[Meta] [New docs] Full-text search

[tkajtoch]

Summary

We want to improve the search experience for our users and have a site-wide full-text search integrated into the new documentation page (EUI+).

Acceptance criteria

• Full-text search should be based on Elastic App Search and have a configured indexing engine to index all contents of the main documentation site (eui.elastic.co)
  • Contents of PR environments (/pr_* paths) should be excluded from being indexed
• It should search in page type (e.g, component or pattern), title, body, metadata (e.g., provided component export name), and other optional custom fields
• It should provide a search field on the site's header and display search results when typing (as "autocomplete")
• The search results should be sorted by relevance and be grouped by types (e.g., component, pattern)
• Every production deployment should trigger site reindexing