docs: transition from rst to myst markdown using rst2myst

[minrk]

Mostly running rst2myst, then fixing various references and occasional formatting errors.

I haven't done a thorough review of formatting problems, but fixed the mistakes I noticed.

The only content change I made was updating the docs/contributing references to rst to trade them for markdown references.

referenced in #838 now that we've released 0.2

closes #753

[consideRatio]

I gave it a passthrough as well and caught a few things now fixed, let's go for a merge!

[jrdnbradford]

Hi @minrk and @consideRatio. I realized that docs/howto/env was removed in this PR when building the docs for something I'm working on. If you look at The user environment section of the TLJH's How To docs, there are no subheadings or information under it since it was removed.

Looks like the files in there were removed in commit https://github.com/jupyterhub/the-littlest-jupyterhub/commit/de36cfc116ee59414d6968abb9a0162a4cd241a5 and include the following:

• docs/howto/env/notebook-interfaces.rst
• docs/howto/env/server-resources.rst
• docs/howto/env/user-environment.rst

Not sure if this was intentional and the The user environment heading should be removed or if the .rst should be re-added as .md.