Migrate `.rst` Document Under `docs/source` to `.md`

[fangjunzhou-nv]

As discussed internally, the team agreed to migrate the document from .rst to .md for better agentic integration.

Currently, all the .rst files are under docs/source.

Related link:

• Sphinx markdown support: https://www.sphinx-doc.org/en/master/usage/markdown.html

Related isse:

#565

[jmccaffrey-nv]

We'll need to analyze how Markdown and RST are used in project, and check that MD has functional parity.
Also quick check to github/slack when Ruilong added RST docs for RST rationale.