Add Markdoc to the doc tools (#11782)
This highlights the test-builds repo,
and contains the basic config to make it build on RTD.
This doesn't address any integration info,
but I wanted to get a basic page up to start getting SEO.
<!-- readthedocs-preview docs start -->
---
:books: Documentation previews :books:
- User's documentation (`docs`):
https://docs--11782.org.readthedocs.build/en/11782/
<!-- readthedocs-preview docs end -->
<!-- readthedocs-preview dev start -->
- Developer's documentation (`dev`):
https://dev--11782.org.readthedocs.build/en/11782/
<!-- readthedocs-preview dev end -->
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Co-authored-by: Manuel Kaufmann <humitos@gmail.com>
Co-authored-by: Anthony <aj@ohess.org>
