[clang-tools-extra] clang-doc switched from using relative to absolute paths (PR #93281)

[Paul Kirth via cfe-commits]

https://github.com/ilovepi requested changes to this pull request.

I'm not completely sure we want to use absolute paths, but since this mostly seems related to URLs and not file system paths, its probably fine.

Please add some tests for this. It's preferable if we can pre-commit tests with the existing behavior, and then can easily see how this patch modifies the old/undesirable behavior.

Lastly, please update the title:
```[clang-doc] Use absolute paths in URLs
```
and update the description to describe what this fixes and how, and links to the GitHub issue using `fixes` or something equivalent from https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue

https://github.com/llvm/llvm-project/pull/93281