docs: add Prompt component page and changelog entry

[kgowru]

Summary

Adds documentation for the new <Prompt> MDX component (implemented in fern-platform #9605):

• New page at component-library/default-components/prompt.mdx — usage, variants (copy-only, single open-in, multiple open-in dropdown), and properties reference
• Navigation entry in docs.yml between "Parameter field" and "Runnable endpoint"
• Changelog entry for 2026-03-26

Updates since last revision

• Added a "How it works" section explaining that markdown formatting in prompt children is automatically preserved when copied or opened in an AI tool (powered by the remarkPromptRawContent remark plugin in fern-platform #9605)
• Updated all examples to use markdown-rich prompt content (bold, links) instead of plain-text URLs, demonstrating the markdown preservation feature
• Updated children property type from string | ReactNode to markdown and clarified that formatting is preserved
• Updated changelog entry to mention markdown preservation

Review & Testing Checklist for Human

• Merge ordering: The fern-platform PR (#9605) with the component implementation must ship before (or alongside) this PR, otherwise the <Prompt> examples won't render
• Verify example links resolve: Examples reference real pages like https://buildwithfern.com/learn/docs/getting-started/quickstart and https://buildwithfern.com/learn/sdks/generators/typescript/quickstart — confirm these URLs are valid
• Preview the page in a deploy preview to check layout, especially the new "How it works" section, code block examples, <ParamField> rendering, and <Markdown src="/snippets/agent-directive.mdx"/> inclusion
• Verify the changelog link /learn/docs/writing-content/components/prompt resolves to the correct page once both PRs are merged

Suggested test plan: After merging fern-platform #9605, deploy a preview of this PR and navigate to the Prompt component page. Verify all example code blocks render the component correctly, the "How it works" section reads well, the navigation link appears in the sidebar, and the changelog entry links back to the page.

Notes

• The component page follows the same structure as existing component pages (Copy, Callout, Icon): frontmatter → agent directive snippet → intro → Usage → Variants → Properties
• The Prompt entry in docs.yml does not set an explicit slug (unlike parameter-fields), so the auto-generated slug from the filename is used

Link to Devin session: https://app.devin.ai/sessions/20a3aeeb0d50425f8e592b8912861201
Requested by: @kgowru

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1774493154-prompt-component-docs.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/changelog/2026/3/26
• learn/docs/writing-content/components/prompt