Make sure a.anchor::after has position absolute all the time

Chad Lavimoniererequested to merge
20250311-clavimoniere-markdown-heading-anchor-position-hover-fix into master

What does this MR do and why?

Make sure a.anchor::after has position absolute all the time

Currently on production, position: absolute; is only applied when the heading is hovered, causing some layout jumpiness.

References

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Before After
Screen_Recording_2025-03-11_at_15.15.15 Screen_Recording_2025-03-11_at_15.15.44

How to set up and validate locally

  1. Check out this branch
  2. Create a new issue in the GDK with the following contents in the description:
### Test metadata

| Field | Value |
| ------ | ------ |
| File URL | [`ee/spec/requests/remote_development/integration_spec.rb#L469`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/spec/requests/remote_development/integration_spec.rb#L469) |
| Filename | `ee/spec/requests/remote_development/integration_spec.rb` |
| Description | `` Full workspaces integration request spec a happy path workspace lifecycle behaves like workspace lifecycle successfully exercises the full lifecycle of a workspace `` |
| Test level | `integration` |
| Hash | `83533d39bc3218161f24b71c92514ffafb0923936` |
| Max expected duration | < 19.2 seconds |
  1. Hover over the Test metadata heading
  2. The layout will not shift when you hover and the anchor appears for the heading

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