Crawling &Indexing content from external website

Notes:

• This will be managed by a specific app,
• Website/Page will be identified by Sitemap,
• Crawling of a sub-page will be based on a local configuration regarding the allowed numbers of hop if the linked page is hosted locally (same domain), on a different subdomain, or on a completely different domains,
• External content have no id within Nextcloud's database, also the crawling of a single address can returns multiple document.

Tasks:

• Allowing an app to directly reach the used Search Platform (fulltextsearch_elasticsearch) without passing through the FullTextSearch index table (core)
• Crawling website and sub-pages (app),
• Extracting content and meta-data for each page (app),
• indexing content and meta-data (app),
• Searching and Advanced Searching within content and metadata (core+app),
• Result should link to the right page (app, might need some work in core to force opening on a different tab)

The issue names no files or tests; begin by tracing the core FullTextSearch index-table path and the fulltextsearch_elasticsearch platform, then identify the app entry points for sitemap crawling. Done means external pages and sub-pages are crawled, content and metadata are indexed and searchable, and results link to the correct page.