Add Unit Testing

[mvorisek]

Tests should not only assert the buid passes, but also the output is exactly what we expect.

The base format of the tests input is markdown with MyST extension. This allows us to test native rst using eval-rst as well as pure AST processing - for future for #55.

When a test input is modified, the GH Actions autocommit the output changes to <current_branch>.changes branch so you can easily fix the expectaions.

[markstory]

The build 'passed' but with errors

Build finished. The HTML pages are in _build/html.
./search.html:20: parser error : Specification mandates value for attribute defer
  <script src="searchindex.js" defer></script>
                                    ^
-:1: parser error : Document is empty

[mvorisek]

The build 'passed' but with errors

Build finished. The HTML pages are in _build/html.
./search.html:20: parser error : Specification mandates value for attribute defer
  <script src="searchindex.js" defer></script>
                                    ^
-:1: parser error : Document is empty

It is a passing xmlint warning, I sent sphinx-doc/sphinx#11494 PR to sphinx repo, for now, ignore the warning, once sphinx will release fixed version, the warning will be gone.