Add API to search index

API part is currently not indexed.

I currently don't see a way to index them properly with the way how API pages are structured (multiple headings as H1, title containing not only name but also type). When migrating to Statiq we should make sure that API docs can be indexed.

Start by reviewing how API pages are structured and how the current search index handles documentation. Revisit the indexing approach during the migration to Statiq, especially pages with multiple H1 headings and titles containing both a name and type. Done means API documentation pages can be indexed and searched correctly.

Written by the indexing model from the issue text.