chore: document adoc anchor and generated-catalog-sync guidance in AGENTS.md

[davsclaus]

Summary

• Adds two AI-agent guidelines to the "Documentation Conventions" section of AGENTS.md:
  1. Avoid explicit [[anchor]] blocks before headings — AsciiDoc already auto-generates an id (_-prefixed) from the heading text, and an explicit anchor silently diverges from that, breaking xref: links that expect the auto-generated id.
  2. Editing a component's src/main/docs/*.adoc requires regenerating the mirrored copy under catalog/camel-catalog/src/generated/resources/.../docs/, or CI's "uncommitted changes" check fails — this applies to doc-only edits too, not just code-driven metadata changes.
• Prompted by fixing a broken structured_error_exchange_properties anchor across several AI component docs, and a follow-up PR (chore: regenerate catalog docs for structured_error_exchange_properties anchor fix #25941) to regenerate the catalog copies that were missed the first time.

Claude Code on behalf of davsclaus

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[gnodet]

Clean, well-motivated addition of two technically accurate guidelines to the Documentation Conventions section, prompted by a real incident where an explicit AsciiDoc anchor broke xref links and a missed catalog regeneration broke CI.

Both guidelines are directly traceable to a real incident chain and are well-worded with concrete examples. No issues found.

📋 PR Metadata

Aspect	Current	Suggested
Milestone	(none)	4.23.0

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Claude Code on behalf of Guillaume Nodet