Documentation: Runner fleet updates

Problem to solve

After discussing in #33803[Project-runners.png] (comment 1037833523), @sselhorn noticed that the docs were outdated and not in an ideal set-up to answer the question How do I configure runners? (which is the link text that links to this https://docs.gitlab.com/ee/ci/runners/index.html). It is not easy to understand what runners are, how to configure them, and how they work by looking at the docs today.

Further details

As part of this issue, I (Suzanne) reviewed the following UI pages:

Project > Settings > CI/CD > Runners

These links had to be fixed:

• How do I configure runners - This page was about SaaS runners, not about self-managed. Also, the UI already had information about installing and registering runners. So I updated this language to be What is GitLab Runner? and I link to the top-level page. Opened MR: !95292 (merged)
• Shared Runners on GitLab.com - This is not the correct link, and "runners" should be lowercase. Need to open infra issue to fix: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/16189
• Group owners can register group runners in the group's CI/CD settings. - Opened MR to update language: !93609 (merged). Link was OK.

These links were OK as-is:

• Install GitLab Runner and ensure it’s running
• Autoscale mode
• GitLab.com plans
• Runner API

Group > Settings > CI/CD > Runners and Group > CI/CD > Runners

All links were OK.

Admin > Overview > Runners

All links were OK.

Admin > CI/CD > Continuous Integration and Deployment

Shared runner details - message was no longer correct. Opened MR to fix: !95289 (merged)

Pipeline editor page, click Help. Drawer opens.

The link for "make sure your instance has runners available" goes to Runner SaaS page. Opened MR to change it to a self-managed page: !95301 (merged)

Who can address the issue

@gdoyle can support!

Other links/references