Trim browser-only chrome from ?output_format=md requests

[jom]

Summary

When a request asks for the markdown rendering (via ?output_format=md, ?output_format=markdown, or an Accept: text/markdown header), the html-to-md mu-plugin converts the full template output to markdown. Several pieces of that output are meaningful in the browser but pure noise in a markdown body intended for LLMs and scripted consumers.

This PR adds a single inc/markdown-output.php whose filters fire only when is_markdown_request() is true, leaving the browser rendering untouched.

What it suppresses on markdown requests

• "In this article" sidebar TOC and its "↑ Back to top" tail (wporg/sidebar-container, wporg/table-of-contents)
• User Contributed Notes — heading, login prompt, comment form, list of notes, edit form (wporg/code-reference-comment-form, wporg/code-reference-comments, wporg/code-reference-user-notes, wporg/code-reference-comment-edit)
• Handbook page footer patterns: article-meta ("First published" / "Last updated" — already in the YAML frontmatter) and handbook-pagination ("Previous:" / "Next:" nav)
• The author YAML frontmatter field (rarely reflects authorship of the rendered content on this site)

What it improves

• wporg/code-table rows past itemsToShow are JS-hidden behind a "Show more" toggle; bumped to PHP_INT_MAX on markdown requests so the full Changelog / Related tables render inline
• Parameter <dt> markup is <code>$var</code><span class="type">…</span><span class="required">…</span> with CSS spacing — inject whitespace between the spans on markdown requests so the line reads `$var` string required instead of `$var`stringrequired

Mechanism

• pre_render_block short-circuits suppressed blocks (and core/pattern by slug) — render callbacks and their queries never run
• render_block_data mutates wporg/code-table's itemsToShow attr
• render_block_wporg/code-reference-parameters injects parameter spacing into the rendered HTML
• html_to_markdown_frontmatter_fields drops the author key

Test plan

• Visit ?output_format=md on a function page — confirm TOC, Back-to-top, and User Contributed Notes are gone; Changelog and Related render every row; parameters read with spaces
• Visit ?output_format=md on a handbook page — confirm no "First published / Last updated" footer or Previous/Next nav
• Visit either page in a browser (no query arg) — confirm no visible change to the rendered page
• Accept: text/markdown header without query arg behaves the same as ?output_format=md

[jom]

This is tied to dmsnell/html-to-md#16 and I'd consider it optional. Depending on timeline of merges, I'm happy to remove from this PR.

[jom]

I was teetering on this change. Happy to revert if we think user comments are useful to the mostly-LLMs consuming them, but I thought it could be a good way to limit tokens.