Improve page metadata for title and description (used in search results and social previews)

[LinqLover]

Previously, these looked very generic for most pages (essentially showing the same text for almost all pages, and just repeating the title twice for release notes). Now they look much more precise. :-)

• If no title is available from the page config, fall back to the title in the section config (global collection) (analogously to how <title> tag is already computed)
• Descriptions can be be specified in the section config or in the frontmatter of pages
• If no description is specified, generate one instead from the first meaningful paragraph of the page
• Fix section generator to properly parse frontmatter from _head.md instead outputting it verbatim

(https://socialsharepreview.com/)

[Copilot]

Pull request overview

This PR improves the precision of page <title>, meta name="description", and Open Graph metadata so search results and social previews better reflect each page/section, including more helpful fallbacks when explicit metadata isn’t provided.

Changes:

• Add and propagate section/page descriptions via _config.yml, page frontmatter, and _head.md frontmatter.
• Update the section generator to parse _head.md frontmatter instead of rendering it as content, and to provide a section-title fallback for metadata.
• Introduce a post-render hook that derives a description from the first meaningful paragraph when none is provided, and patches rendered meta tags accordingly.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
index.html	Reuses the welcome text as the page description via YAML anchor/alias to improve homepage metadata.
_plugins/section_generator.rb	Parses _head.md frontmatter, sets section-level description/title fallback flags for downstream metadata generation.
_plugins/page_metadata.rb	Adds description extraction and post-render meta tag rewriting for more accurate description and Open Graph tags.
_pages/terse_guide.md	Adds an explicit description in frontmatter for better previews.
_pages/board.md	Adds an explicit description in frontmatter for better previews.
_downloads/_head.md	Adds frontmatter description for the downloads section (now correctly parsed by the generator).
_config.yml	Adds a description for the documentation collection to provide a section-level fallback.

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