ci: extend doc-linter to MyST Markdown

Active docs are now MyST `.md`; the linter previously only inspected
`.rst` and `.txt`, so ~250 active pages were unchecked for IP usage and
line length on every PR.

scripts/doc-linter.py:
- Add `.md` to the extension filter (use `endswith` for correctness;
  the prior 4-char slice silently skipped `.md` files).
- Track MyST/Markdown fenced code blocks (```` ``` ```` and `:::`) for
  line-length exemption — same semantics as `.. code-block::` for RST.
- Recognize both suppression marker forms: `.. stop_vyoslinter` /
  `.. start_vyoslinter` (RST and `.txt` includes) and `% stop_vyoslinter`
  / `% start_vyoslinter` (MyST). Both work in either context; pick the
  form that matches the surrounding parser.
- Replace the brittle `try/finally: fp.close()` with a `with` block —
  the previous form raised `UnboundLocalError` if `open()` itself
  failed.
- Fix typo `forgett` → `forget`.

.github/instructions/rst-linter.instructions.md → doc-linter.instructions.md:
- Broaden `applyTo` from `**/*.rst` to `**/*.md,**/*.rst,**/*.txt`.
- Document MyST suppression syntax and fenced-code line-length
  exemption.
- Note the parser-form rule for `{eval-rst}` blocks.

No regression on `.txt` includes: identical lint output verified
against the origin/rolling baseline on a sample of files.

Pre-existing IP violations exist in 14 `.md` files (e.g.
`configexamples/lac-lns.md` line 95 — a `8.8.8.8` already wrapped in
`% stop_vyoslinter`/`% start_vyoslinter`, correctly suppressed). PRs
touching unsuppressed violations will start failing CI; this is the
intent of enabling the check.

🤖 Generated by [robots](https://vyos.io)

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