Add :highlights: and keep-highlight-descriptions to changelog directive

[lcawl]

Goal

Highlights are a boolean flag (highlight: true), not an entry type.
The changelog directive should stop treating them as a :type: value.
This PR enables authors to put a "Highlights" section on any page that uses the directive (and optionally show their full descriptions even when none of the other sections show them)

Implementation details

• Adds a new :highlights: option for the changelog directive
• Adds a new :description-visibility: keep-highlight-descriptions option
• Removes the highlights value from the :type: option

Code

• :highlights: boolean on ChangelogBlock via PropBool("highlights"), threaded through ChangelogRenderOptions
• Removed ChangelogTypeFilter.Highlight; legacy :type: highlight warns and falls back to Default
• Highlights section gated on HighlightsEnabled (not :type: all); TOC/anchors updated to match
• Added :description-visibility: keep-highlight-descriptions:
  • Shows record descriptions only in the Highlights section
  • Hides them in all other sections (including the Features copy of a highlighted entry)
  • Without :highlights:, hides descriptions everywhere

Docs / fixtures

• Updated docs/syntax/changelog.md and docs/data/release-notes/publish.md
• docs-tests release-notes pages now use :highlights:

Tests

• Added ChangelogHighlightsOptionTests.cs

Screenshots

When I add a directive like this in the Cloud Serverless release notes page:

:::{changelog}
:cdn: cloud-serverless
:highlights:
:description-visibility:  keep-highlight-descriptions
:link-visibility: keep-links
:::

The rendered page contains highlight sections like this:

And another:

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Cursor Grok 4.5