DEVX-118: feat: enrich lint_docs.py with single H1, max depth, line length, code block lang, orphan checks

- Add check_single_h1: each markdown file should have at most one H1
- Add check_max_heading_depth: headings should not exceed H4 (configurable)
- Add check_line_length: warn on lines >120 chars (non-blocking — badge URLs)
- Add check_code_block_languages: fenced code blocks must specify a language
- Add check_orphan_docs: warn on docs not linked from index.md or mapping.json
- Fix all code blocks in docs to specify language (text for plain blocks)
- Fix duplicate H1 in .vale/styles/devx/README.md
- Add 18 new tests for full coverage of new checks

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

docs/user/getting-started.md

@@ -72,7 +72,7 @@ tea CLI, etc.) and configure pre-commit hooks.
 
 devx expects a `docs/` directory with at minimum:
 
-```
+```text
 docs/
 ├── index.md          # Documentation home page
 ├── mapping.json      # Wiki page title mappings