Fix odoc errors in documentation
- Fix ambiguous {!decode} and {!connection} references by using val- prefix
- Add section IDs to header_parsing.mli and fix cross-references to sections
- Escape curly braces in documentation examples to avoid bad markup errors
- Fix unpaired brace in URL reference in headers.mli
- Indent verbatim block content in mqtte_cmd.mli
Unresolved references to external libraries (Jsont, Tomlt_bytesrw, etc.)
are intentionally kept as cross-references for future resolution.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>