Add support for reStructuredText

Hi, the [Pigweed](https://pigweed.dev) team is interested in using stardoc. One problem: we use [Sphinx](https://www.sphinx-doc.org) and write our docs in [reStructuredText](https://docutils.sourceforge.io/rst.html). Could you all implement reST support? Or would you be open to a contribution that adds reST support? Sphinx is a very popular tool for generating docs sites, and by extension reST is fairly popular, so I think it could be worthwhile to invest in reST support.

Start by reviewing Stardoc’s existing documentation-generation pipeline and how it represents output formats; the issue names no files or tests. Define the scope needed for reStructuredText output that works with Sphinx, then add coverage demonstrating that support and verify the generated documentation can be consumed by Sphinx.

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