docs(readme): reflect completed MyST migration

The MyST migration is complete: source_suffix in docs/conf.py is now
['.md'] only, all canonical pages are .md, and the pre-migration RST
originals are archived under docs/_rst_legacy/ (excluded from the
build). Update the README accordingly:

- Add a 'Source format' section stating pages are MyST Markdown and
  pointing at the _rst_legacy/ archive so newcomers know not to edit
  it. Mention that the VyOS command directives are written as MyST
  fenced blocks via myst_fence_as_directive, and that docs/_include/
  *.txt snippets stay RST because cmdincludemd parses their content
  as RST (per 9c815d6).
- Drop the stale 'MyST Markdown for migrated pages, RST for the rest'
  line from the Contributing section; rephrase to point at the actual
  topics AGENTS.md covers today (MyST conventions, command directives,
  linter suppression markers, Copilot/CodeRabbit workflow).

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