docs: convert reStructuredText sources to MyST markdown

[timsaucer]

Which issue does this PR close?

There is no open issue but this continues the work done in #1578.

Rationale for this change

Phase 2 of the documentation-site refresh started in #1578. With the modern pydata-sphinx-theme + navigation in place, this PR moves the content format off .rst and onto MyST .md. The motivation:

• Markdown is the lingua franca of agent-tuned tooling. LLMs trained on GitHub and modern docs parse Markdown reliably; reStructuredText is a minority dialect that frequently confuses both humans editing via PR review and agents reading the source. The Apache datafusion-comet sibling project completed the same migration recently and reported smoother contributor onboarding.

What changes are included in this PR?

• Format conversion (mechanical, via rst-to-myst).
• Manual fixes layered on top of the converter output for cross references
• AGENTS.md is updated so the two .rst paths called out under "Aggregate and Window Function Documentation" point at the new .md equivalents.
• Switched from myst-parser to myst-nb so that we can do markdown parsing PLUS code execution to render our examples.

Are there any user-facing changes?

No behavioral change to the datafusion package — only the source format of the published documentation. Readers of the rendered site will not notice the migration; the HTML output is slightly updated but still shows all of the relevant content including running code.

Follow-ups (out of scope for this PR)

• Phase 3: multi-version doc publishing (the comet pattern).
• Phase 4: asf-site publishing workflow.