Add subscription troubleshooting page

What does this MR do?

Adds a troubleshooting page for GitLab.com and self-managed subscription, because some errors were duplicated on the two pages, and the .com troubleshooting section had five topics so it required a new page.

Related issues

• CTRT - Gitlab.com Subscription (#476443 - closed)
• technical-writing#1078
• technical-writing#1075

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.

doc/subscriptions/gitlab_com/gitlab_subscription_troubleshooting.md

@@ -7,17 +7,20 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 
 # Troubleshooting GitLab subscription
 
-When you use GitLab subscriptions, you might encounter the following issues.
+DETAILS:
+**Tier:** Premium, Ultimate
+**Offering:** GitLab.com, Self-managed
+
+When you use subscriptions for GitLab.com or GitLab self-managed, you might encounter the following issues.
 
 ## Credit card declined
 
-If your credit card is declined when purchasing a GitLab subscription, possible reasons include:
+When purchasing a GitLab subscription, your credit card might be declined because:
 
-- The credit card details provided are incorrect. The most common cause for this is an incomplete or fake address.
+- The credit card details are incorrect. The most common cause for this is an incomplete or fake address.
 - The credit card account has insufficient funds.
-- You are using a virtual credit card and it has insufficient funds, or has expired.
-- The transaction exceeds the credit limit.
-- The transaction exceeds the credit card's maximum transaction amount.
+- The credit card has expired.
+- The transaction exceeds the credit limit or the card's maximum transaction amount.
 
 Check with your financial institution to confirm if any of these reasons apply. If they don't
 apply, contact [GitLab Support](https://support.gitlab.com/hc/en-us/requests/new?ticket_form_id=360000071293).
@@ -25,10 +28,10 @@ apply, contact [GitLab Support](https://support.gitlab.com/hc/en-us/requests/new
 ## Error: `Attempt_Exceed_Limitation`
 
 When purchasing a GitLab subscription, you might get the error
-`Attempt_Exceed_Limitation - Attempt exceed the limitation, refresh page to try again.`
+`Attempt_Exceed_Limitation - Attempt exceed the limitation, refresh page to try again.`.
 
 This issue occurs when the credit card form is re-submitted three times within one minute or six times within one hour.
-To resolve this issue, wait a few minutes and try the purchase process again.
+To resolve this issue, wait a few minutes and retry the purchase.
 
 ## Error: `Subscription not allowed to add`
 
@@ -44,22 +47,21 @@ To resolve this issue, contact your [GitLab sales representative](https://about.
 
 ## No purchases listed in the Customers Portal account
 
-To view purchases in the Customers Portal, in the **Subscriptions & purchases** page,
-you must be a contact in your organization for the subscription.
+To view purchases in the Customers Portal on the **Subscriptions & purchases** page,
+you must be added as a contact in your organization for the subscription.
 
 To be added as a contact, [create a ticket with the GitLab Support team](https://support.gitlab.com/hc/en-us/requests/new?ticket_form_id=360000071293).
 
 ## Unable to link subscription to namespace
 
-If you cannot link a subscription to your namespace, ensure that you have the Owner role
-for that namespace and review the [transfer restrictions](index.md#transfer-restrictions).
+If you cannot link a subscription to your namespace, you might have insufficient permissions.
+Ensure that you have the Owner role for that namespace, and review the [transfer restrictions](index.md#transfer-restrictions).
 
 ## Subscription data fails to synchronize
 
 On GitLab self-managed, your subscription data might fail to synchronize.
-
 This issue can occur when network traffic between your GitLab instance and certain
 IP addresses is not allowed.
 
-To resolve this issue, allow network traffic from your GitLab instance to IP
-addresses `172.64.146.11:443` and `104.18.41.245:443` (`customers.gitlab.com`).
+To resolve this issue, allow network traffic from your GitLab instance to the IP addresses
+`172.64.146.11:443` and `104.18.41.245:443` (`customers.gitlab.com`).