Serve theme documentation from this repository and add PR previews

[choldgraf]

Currently, we store the documentation for our bundled myst themes in the mystmd repository. The documentation in this repository is a demo site of components for references.

By having the documentation for the theme in a separate repository, we have a few downsides:

• Changes in functionality to this theme will require changes to documentation in a different space (mystmd.org).
• Changes in functionality to this theme aren't easily previewed in PRs, because no local documentation is built as a preview.

I propose that we do the following things:

• move all documentation that is related to the bundled myst themes into this repository (in docs/)
• move the components site to a sub-directory like docs/components/
• add a netlify PR preview for the documentation that uses the latest theme build in this repository
• keep templating/theme documentation in mystmd if it is about web templating in general, and not these themes specifically.
• embed the documentation for these themes into the mystmd.org docs so that it functionally looks the same that it does now (perhaps with "myst default themes" having a dedicated page)