Migrate to Mkdocs

[gonzalocasas]

Migrate to mkdocs, add typing hints everywhere, removed some older stuff

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas.datastructures.Mesh.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[gonzalocasas]

@chenkasirer ping ;)

[chenkasirer]

LGTM but there's still a couple small things missing, I think: the docs workflow needs to be updated to trigger the mkdocs one and if we want to keep references to the old sphinx sites, we need to run the migration script: https://gist.github.com/chenkasirer/d25103d163e7c2868c39ce6f3df8f79c

[chenkasirer]

aren't a lot of these now conditional dependencies of compas_invocations2?

[gonzalocasas]

done!