change .readthedocs.yaml to switch publishing docs from rst to markdown

[wm-wm-wm]

Change Summary

Switch documentation publishing from RST to markdown

Related issue number

[DOP-37885]

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[github-actions]

Coverage Report •

File	Stmts	Miss	Branch	BrPart	Cover	Missing
syncmaster/server/settings/server
static_files.py	17	9	6	6	47%	33–41
TOTAL	5368	642	832	174	88%

[dolfinus]

We should also update:

• towncrier configs
• changelog check in CI
• Makefile jobs for building documentation (sphinx -> mkdocs)
• CONTRIBUTING.rst (probably should be renamed to .md and used as a snippet)
• drop docs dependency group from pyproject.toml

See MTSWebServices/onetl@d708d99

[dolfinus]

There are 3 .rst files left in the repo:

• README.rst
• SECURITY.rst
• CONTRIBUTING.rst

They should probably be replaced with .md files as well. Also I don't see why we need Security and Contributing sections in mddocs - these files are developer-only, it's enough to store them just in the repo root.

This can be changed in another PR.