Streamline the pipeline docs

I'd like to make some changes to the pipeline content so that the quota information is:

• no longer duplicated (see this and this), and
• closer in the global nav to the other pipeline content.

Here is a proposal.

1. Create a topic under Pipelines in the global nav. Call it Pipeline quota.

   • The topic will be specific to GitLab.com only (self-managed doesn't have these limits unless they're self-imposed).
   • The topic will contain these sections from the Customer Docs topic.
     • CI pipeline usage quota
     • How the pipeline usage quota is calculated
     • Buy extra shared pipeline runner minutes - Two current topics -
       • https://docs.gitlab.com/ee/subscriptions/index.html#extra-shared-runners-pipeline-minutes and
       • https://docs.gitlab.com/ee/subscriptions/index.html#purchasing-additional-minutes
     • What happens when minutes run out

2. When I remove the topics from the Customer Docs topic, in its place I will add information that says your CI pipeline depends on the tier you buy, and point to the new pipeline topic for details. (This Customer Docs topic is about how you choose whether to do SaaS or self-managed. I will also say if you do self-managed you can set a limit and link to this topic/heading).

3. I will update the Gitlab.com settings topic to point to the new pipeline topic. (There is a Linux Shared Runners topic that mentions the quota.)

Throughout, I will also standardize on some terms. In the UI it says group pipelines quota, personal pipelines quota, and pipelines quota. It also says Buy more CI minutes.

Here are ways it's currently described in the docs:

• Usage quota
• Shared Runners pipeline minutes quota
• Shared Runners pipeline minutes
• pipeline duration
• CI minutes
• CI pipeline minutes

Also, in the docs it doesn't mention Personal pipelines quota anywhere, so need to add this. It looks like it shows all the groups you have permission to. And it links to this. I should probably update that topic to point to the new Pipeline quota topic I plan to create.

Need to make sure I update all links in the product and other broken links that happen when I move these topics.

There are strange places that the UI is linking right now--

In the user interface, if you look at the personal quota and click the ?, it brings you to this page: https://gitlab.com/help/subscriptions/index#extra-shared-runners-pipeline-minutes

If you look at the group quota and click ?, it brings you here: https://gitlab.com/help/user/admin_area/settings/continuous_integration#shared-runners-build-minutes-quota