Converts search to new Pagefinder

[BoDonkey]

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Summary

Summarize the changes briefly, including which issue/ticket this resolves. If it closes an existing Github issue, include "Closes #[issue number]"
This PR updates the doc search to use the Pagefinder package. This was needed to support the marketing site unified search results. Rather than maintaining two packages (the old search and the new Pagefinder) the docs search was converted. Closes PRO-9750

What are the specific steps to test this change?

Full details: SEARCH.md at the repo root.

1. rm -rf node_modules package-lock.json && npm install (a rollup version pin was added —
   a clean install picks it up; see README.md if you hit a swagger-ui-dist build error)
2. npm run build (this also builds the Pagefind index — build:search-index runs
   automatically as part of build)
3. npm run preview
4. Open the preview site, click the search icon, and confirm:
   • Search returns results with highlighted excerpts
   • The "Ask AI" toggle still works as before
   • Search does not return any marketing-site (apostrophecms.com) results — that's
     correct, not a bug
5. Search does not work in npm run dev / vitepress dev — the box will say "Search is
   unavailable right now." That's expected; the index is a build artifact.

For example:

1. Run the website and log in as an admin
2. Open a piece manager modal and select several pieces
3. Click the "Archive" button on the top left of the manager and confirm that it should proceed
4. Check that all pieces have been archived properly

What kind of change does this PR introduce?

(Check at least one)

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[linear]

PRO-9750

[boutell]

LGTM