Add release-dates option to changelog directive

[lcawl]

Background

@itsalexcm determined in his research for the new release notes site that there's value to having dates associated with all of our releases.
With the current stop-gap method of publishing release notes via changelog directives, release dates are always rendered if they exist (e.g. in EDOT Java release notes.
However, showing release dates for date-based releases (where the date already appears in the title) currently makes little sense.
Therefore, this PR separates the existence of release date info in the bundle from whether/not it's rendered.

Goal

Add opt-in :release-dates: on {changelog} (aligned with :subsections: / :dropdowns:). Render-time only; no CLI changes.

Design

Same pattern as existing boolean options:

Subsections = PropBool("subsections");       // omitted → false
DropdownsEnabled = PropBool("dropdowns");    // omitted → false
ReleaseDatesEnabled = PropBool("release-dates");  // omitted → false

Directive usage	Behaviour
(omitted)	Do not render _Released:_
:release-dates: or :release-dates: true	Render _Released:_ when bundle has release-date

Breaking change: pages that currently show release dates must add :release-dates:.
For example: elastic/elastic-otel-java#1149

Code

• ChangelogBlock.cs — ReleaseDatesEnabled via PropBool("release-dates") (default false, same as :subsections: / :dropdowns:)
• ChangelogInlineRenderer.cs — passes bundle.Data?.ReleaseDate to the header only when ReleaseDatesEnabled is true

Tests

• ChangelogBasicTests.cs — existing release-date tests now use :release-dates:
• ChangelogReleaseDatesOptionTests.cs — new coverage for default-off, enabled, missing bundle date, and description still rendering

Docs

• docs/syntax/changelog.md — options table, example, :release-dates: section, CDN note, and rendered-output guidance (bundle-time vs render-time)

Breaking change: pages that show _Released:_ must add :release-dates: to the directive.

Generative AI disclosure

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Tool(s) and model(s) used: composer-2.5-fast

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Enterprise

Run ID: 7253ce95-d126-4f6c-8132-e72c8a6bdf5c

📥 Commits

Reviewing files that changed from the base of the PR and between 10a14b5 and 12244b0.

📒 Files selected for processing (6)

• docs/syntax/changelog.md
• src/Elastic.Markdown/Myst/Directives/Changelog/ChangelogBlock.cs
• src/Elastic.Markdown/Myst/Directives/Changelog/ChangelogInlineRenderer.cs
• src/Elastic.Markdown/Myst/Directives/Changelog/ChangelogRenderOptions.cs
• tests/Elastic.Markdown.Tests/Directives/ChangelogBasicTests.cs
• tests/Elastic.Markdown.Tests/Directives/ChangelogReleaseDatesOptionTests.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch changelog-directive-dates

• 🛠️ Update Documentation

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[cotti]

I've tweaked a couple of calls that had parameter creep. Otherwise, LGTM!