Fix links in documentation

[yangdanny97]

Summary of changes:

• fix anchor names/capitalization in links (they are always kebab case, lowercased)
• change relative references to point to .ipynb (.html relative references do not generate correctly, it seems)
• move Contributing.md contents into Contributing.ipynb. If we {include} the former into the latter, the documentation links will not work due to the extra ./docs/ prefix. I don't think there's an easy way to write links that will work for both, so I just moved it.

References

closes #1132

Chores

• linted
• tested (python scripts/docs.py and clicked around the built docs)
• documented
• changelog entry

[github-actions]

👈 Launch a binder notebook on branch yangdanny97/jupyterlab-lsp/main

[yangdanny97]

CI seems kind of noisy, not sure what to make of it

[krassowski]

Thank you!

[krassowski]

Yes, we need to fix some tests or mark them as XFail for now.