[docs] Add "Copy page as markdown" button

[dokson]

Summary

Fixes #7687.

Adds a "Copy page" button to every docs page that hands the reader an LLM/IDE-friendly markdown blob of the current page, like Radix and Groq do.

Pipeline (docs-data)

For every page produced by the MarkdownPlugin, the build now reads the source .mdx via PageData.sourcePath, runs it through a new stripDocumentalistTags helper, and attaches the result as sourceMarkdown on the page object. The helper replaces top-level @reactDocs / @reactExample / @interface / @css lines with HTML comment placeholders (<!-- Interactive widget: ... (see online docs) -->) so the exported markdown stays valid and self-documenting. Lines inside fenced code blocks are left untouched, so Sass @use / @import in code samples are preserved.

stripDocumentalistTags lives in its own module (markdownExport.mts) with a dedicated markdownExport.test.ts, so importing it for testing does not trigger the top-level build code in compile-docs-data.mts.

UI (docs-theme)

A new CopyPageMarkdownButton renders a Blueprint AnchorButton with a clipboard icon and "Copy page" label. On click it writes sourceMarkdown to the clipboard and swaps the icon to a tick with a "Copied!" tooltip for 1.5s. The button is hidden when sourceMarkdown is undefined (e.g. transient pages or build-time skip).

Wiring (docs-app)

BlueprintDocs.renderPageActions now renders the new button alongside the existing "Edit this page" link, so every page action area gets both controls.

Test plan

• pnpm --filter @blueprintjs/docs-data run test for markdownExport.test.ts — 3/3 passed (existing failures in navHelpers.test.ts are pre-existing and unrelated)
• ESLint / Prettier on changed files
• Manual smoke check in the docs app (deferred to reviewer — local Windows env hits an unrelated docs-data:compile issue with Node 25; the repo's .nvmrc pins 24.14.1)

Out of scope

A "View page as markdown" link or the dropdown variant from the issue's second screenshot. Once the basic sourceMarkdown plumbing is in place those are easy follow-ups, happy to do them in a separate PR if maintainers want.

[changelog-app]

Generate changelog in packages/docs-theme/changelog/@unreleased

Type (Select exactly one)

• Feature (Adding new functionality)
• Improvement (Improving existing functionality)
• Fix (Fixing an issue with existing functionality)
• Break (Creating a new major version by breaking public APIs)
• Deprecation (Removing functionality in a non-breaking way)
• Migration (Automatically moving data/functionality to a new system)

Description

[docs] Add "Copy page as markdown" button

Check the box to generate changelog(s)

• Generate changelog entry

[dokson]

@cameronjoyner — implements #7687 starting from your "Copy page" idea. The sourceMarkdown is plumbed through the docs-data build (with Documentalist tags replaced by HTML comments so the output is valid markdown for LLM/IDE tooling), and the button lives in @blueprintjs/docs-theme next to the existing CodeExample widget so external docs consumers can reuse it. Happy to follow up with a "View as markdown" link or the dropdown variant if you want them in a separate PR.