Migrate from sphinx to mkdocs

[pavelzw]

Edited by @kklein

This PR migrates datajudges documentation engine from sphinx to mkdocs.

Motivation

• Most of our other QuantCo open source projects have migrated -- relying on the same technology might ease knowledge sharing and maintenance.
• Mkdocs relies on markdown while sphinx relies on rst files. The former is considered to be more popular among developers.
• The general trend of the ecosystem seems to go (or rather have gone) towards mkdocs.

Changes

• Every standalone documentation page has been transformed from an .rst file to a .markdown file.
• The configuration for readthedocs.org has been adapted.
• Cross-references and math in doc strings has been adapted.
• Since the mechanism of deciding what appears in the auto-generated API docs is quite different now, it seemed to be easiest to move some logic from db_access to dedicated modules and to explicitly exclude db_access from the api documentation. Prior to this PR we didn't generate any documentation for it either since it is really not meant to be used by end users.
• Some classes, methods, functions and constants have been marked as private via a _ suffix. Private objects are configured to not appear in the auto-generated api documentation.

Comparison

before
after

[codecov]

Codecov Report

❌ Patch coverage is 95.96929% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.38%. Comparing base (0416325) to head (1034ebc).

Files with missing lines	Patch %	Lines
src/datajudge/constraints/date.py	86.95%	6 Missing ⚠️
src/datajudge/constraints/numeric.py	89.74%	4 Missing ⚠️
src/datajudge/constraints/base.py	94.44%	3 Missing ⚠️
src/datajudge/requirements.py	97.22%	3 Missing ⚠️
src/datajudge/condition.py	92.59%	2 Missing ⚠️
src/datajudge/data_source.py	96.22%	2 Missing ⚠️
src/datajudge/constraints/row.py	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #238      +/-   ##
==========================================
+ Coverage   92.27%   92.38%   +0.11%     
==========================================
  Files          18       21       +3     
  Lines        2058     2062       +4     
==========================================
+ Hits         1899     1905       +6     
+ Misses        159      157       -2     

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[kklein]

@pavelzw I took the liberty of moving this forward. Your feedback is ofc very welcome!