Add option to disable section wrapping in HTML output

[dereuromark]

Document-level headings are wrapped in <section> elements by default, with the heading id hoisted onto the wrapper. There was no way to turn this off short of post-processing the HTML.

That is a real migration blocker: stylesheets written against a flat Markdown heading structure break silently. The motivating case is sibling selectors such as .stack > * + *, which stop matching once every heading and its content sit in their own wrapper. Raised in markup-carve/carve#427 and before in jgm/djot#221 .

What this adds

An opt-out. The default is unchanged.

$converter = new DjotConverter(sections: false);
// or
$converter->getHtmlRenderer()->setSections(false);

djot convert doc.djot --no-sections

{#install .featured}
## Setup

	output
default	<section id="install"> / <h2 class="featured">Setup</h2>
sections: false	<h2 id="install" class="featured">Setup</h2>

Why it is small

renderHeading() already emits exactly this shape. It is the path used today for headings nested inside divs and blockquotes. Disabling section wrapping routes document-level headings through that same path, so the behavior is one that already existed and was already covered, just not reachable at document level.

Two things worth calling out:

• No id loss. The explicit id reservation pass still runs first, so generated ids and their dedup suffixes are byte-identical in both modes. This is not true of the equivalent workaround in djot.js, where the id is moved onto the section during parsing, so unwrapping the section with a filter silently drops every anchor including hand-written ones. Because djot-php decides placement at render time, there is nothing to recover.
• Scope. Section wrapping is an HTML concern. The footnote endnotes section, the round-trip abbreviation metadata, and the plain text, Markdown and ANSI renderers are all unaffected.

Docs

Added to docs/guide/parser-options.md, docs/guide/converters.md, docs/reference/api.md, docs/reference/cli.md, and a migration note in docs/cookbook/markdown.md, which is where someone hitting this problem is most likely to look.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.40%. Comparing base (5c29c6e) to head (7ebab1e).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #266      +/-   ##
============================================
+ Coverage     92.39%   92.40%   +0.01%     
- Complexity     3668     3673       +5     
============================================
  Files           109      109              
  Lines         10407    10421      +14     
============================================
+ Hits           9616     9630      +14     
  Misses          791      791              

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