docs: explain intentionally empty HTML title-block partial

[michaelzehetleitner]

Summary

This documents why _extensions/apaquarto/title-block.html is intentionally empty.

Why

The HTML format still references title-block.html as a template partial. If markup is added there without realizing that frontmatter.lua generates the APA HTML title block, it can silently override the generated front matter.

Change

• keep title-block.html empty in behavior
• add an explanatory comment so future edits do not accidentally override the Lua-generated title block

Notes

This is a documentation-only change. It does not change runtime behavior.

Assistance note: I used AI coding tools to help draft and organize this patch. I reviewed the changes, validated the behavior locally, and take responsibility for the final PR.