4873: Resolve links in hover documentation r=matklad a=zacps
This PR resolves links in hover documentation. Both the upcoming intra-doc-links style and the old "path-based" style.
## Todo
* [x] More tests
* [ ] Benchmark (Is there an easy way to benchmark this?)
* [x] ~~Resolve issues with the markdown parser/get rid of it~~ Migrate to `pulldown_cmark_to_cmark`
* [x] Reorganise code (Tips appreciated)
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Fixes#503
Co-authored-by: Zac Pullar-Strecker <zacmps@gmail.com>
96cbad9fb5