Fix markdown formatting in dropdown titles

[lcawl]

Fixes #3637

Screenshots

Before

After

Summary

AdmonitionBlock.cs — after substitutions, dropdown titles are converted to plain text:

if (Admonition is "dropdown")
    Title = Title.StripMarkdown();

This uses the existing StripMarkdown() helper (Markdig ToPlainText). It applies to:

• Hand-authored {dropdown} directives
• Changelog-generated dropdowns (re-parsed through the normal directive pipeline)
• CLI changelog render output on the next docs build

No changelog renderer changes.

Tests added

File	Coverage
AdmonitionTests.cs	Backticks, **bold**, _italic_ stripped from title and HTML
ChangelogDropdownsTests.cs	ChangelogDropdownsPlainTextTitleTests — known-issue entry with `ElasticAgentVersion`

Docs updated

• docs/syntax/dropdowns.md — new Plain-text titles section
• docs/syntax/changelog.md — cross-reference under :dropdowns:

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: composer-2.5-fast