docs: add production docs link to dev-mode search disabled notice

[Copilot]

In dev/PR-preview builds, Starlight shows "Search is only available in production builds." with no actionable next step — a dead end for anyone exploring via a PR preview.

Changes

• docs/src/components/CustomSearch.astro — New Starlight Search component override. Identical to the built-in except the DEV-mode notice now includes a link to the production docs site:

  {import.meta.env.DEV ? (
    <div style="margin: auto; text-align: center; white-space: pre-line;" dir="ltr">
      <p>{Astro.locals.t('search.devWarning')}</p>
      <p>
        <a href={PRODUCTION_DOCS_URL} aria-label="Search the production documentation site">
          Search the production docs →
        </a>
      </p>
    </div>
  ) : ( /* pagefind UI */ )}

• docs/astro.config.mjs — Registers the override under components.Search.