Resolve "User-generated permalink IDs collide with GitLab interface"
## What does this MR do?
Prevents ID values automatically generated by headers in [GitLab Flavored Markdown](https://github.com/gitlabhq/gitlabhq/blob/master/doc/user/markdown.md#header-ids-and-links) from colliding with IDs used elsewhere in the GitLab interface. This can cause confusion when, for instance, a selector looks for a merge request tab with `id="pipelines"` and there is a header with the same ID earlier in the DOM.
How this works:
* All header IDs generated with GitLab Flavored Markdown are namespaced with `id="user-content_foo"`
* All anchor links which point to these IDs continue to use the non-namespaced hash `<a href="#foo">...</a>`
* When a page is loaded or when the `hashchange` event is triggered, javascript will automatically search for `#user-content_foo` if `#foo` cannot be found, and scroll to that position instead.
## Before
## After:
## Does this MR meet the acceptance criteria?
- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- Tests
- [x] Added for this feature/bug
- [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
## What are the relevant issue numbers?
Closes#22781
See also prior attempts to address this issue:
#3908, !2023, !2024
See merge request !7631
4d37be0fb8