Document merge request status checks widget

What does this MR do?

This MR adds documentation to help explain what the status check widget shows, what states it has and common errors.

It also updates the status check widgets Learn more link to point at the status checks documentation

Screenshots (strongly suggested)

No visual changes

Setup & Testing

1. You must be logged in to view the status checks reports.
2. You need a GitLab Ultimate license.
3. Enable the development feature flag: echo "Feature.enable(:ff_external_status_checks)" | rails c
4. Add a status check to Project > General Settings > Merge Requests > Status checks.
5. Create an MR for the project and view the MR page.

How to create an approved status check?

You can approve a status check via the API !61300 (merged) or by using this code in rails c:

FactoryBot.definition_file_paths = [Rails.root.join('ee', 'spec', 'factories')]
FactoryBot.find_definitions

# Replace with your MR title
merge_request = MergeRequest.find_by_title('Repellendus vero quo rerum atque voluptatem temporibus odit quod corrupti.')

# Create the status check
rule = FactoryBot.create(:external_status_check, project: merge_request.project)
# Approve the status check
FactoryBot.create(:status_check_response, external_status_check: rule, merge_request: merge_request, sha: merge_request.source_branch_sha)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #327634 (closed)