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>

.vale/styles/devx/README.md

@@ -1,2 +1,3 @@
 # Custom Vale style for devx documentation
-# Project-specific terminology and style rules
+
+Project-specific terminology and style rules

.vale/styles/write-good/README.md

@@ -2,7 +2,7 @@ Based on [write-good](https://github.com/btford/write-good).
 
 > Naive linter for English prose for developers who can't write good and wanna learn to do other stuff good too.
 
-```
+```text
 The MIT License (MIT)
 
 Copyright (c) 2014 Brian Ford