[docs] Add CodeToggle widget; use it for install snippets

[dokson]

Summary

Fixes #7684.

Adds a tabbed code-snippet toggle to the docs and uses it for the install commands on the Blueprint and Getting Started pages, so a reader can pick their package manager (npm / yarn / pnpm) instead of the docs hard-coding one.

Architecture

The widget is split in two:

• @blueprintjs/docs-theme → <CodeToggle> — generic, reusable. Takes tabs: { id, label, code }[], an id for the underlying <Tabs>, and an optional storageKey for cross-page persistence in localStorage. Uses Blueprint <Tabs>, <Pre>, <Code>. Sits next to the existing CodeExample widget so any consumer of the docs theme can reuse it.
• docs-app → <InstallSnippet> — thin wrapper that supplies the npm/yarn/pnpm install commands for @blueprintjs/core react react-dom, registered as a @reactDocs InstallSnippet Documentalist tag.

Why the split: Documentalist's @reactDocs tag does not accept arguments, so each specific use case needs its own thin wrapper component registered in tags/reactDocs.ts. The generic CodeToggle lives in docs-theme where it's reusable.

Extensibility

• New package manager (e.g. bun): one entry in the TABS array in installSnippet.tsx. Tab order is the array order.
• New snippet toggle elsewhere (e.g. Vite vs Webpack vs Parcel): a new thin wrapper that calls <CodeToggle tabs={...} id="..." />, re-exported via tags/reactDocs.ts, then @reactDocs MyNewWidget in the relevant MDX page.

Persistence

InstallSnippet passes storageKey="bp-docs-pm-preference", so the reader's PM choice is remembered across pages and across visits. Default is pnpm (the docs current default after #7685). The state hydrates from localStorage after mount to avoid first-render mismatches and stays SSR-safe.

Test plan

• eslint clean on changed files
• prettier applied
• pnpm --filter @blueprintjs/docs-theme run compile:esm clean
• Bundle docs-app and inspect the rendered widget in the browser (deferred — local Windows env hits an unrelated docs-data:compile issue with Node 25; the repo's .nvmrc pins 24.14.1)

No new tests: neither docs-theme nor docs-app has a test setup today, and adding the infrastructure for a single widget felt like scope creep for this issue. Happy to follow up in a separate PR if maintainers want test coverage for docs-theme widgets.

[changelog-app]

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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 CodeToggle widget; use it for install snippets

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[dokson]

@mm-wang — implements the toggle from #7684 as a reusable CodeToggle widget in @blueprintjs/docs-theme. The Blueprint and Getting Started pages now use it for the install command. See PR description for the architectural choices and how to extend (more package managers, or a different toggle for some other code section). Feedback welcome.