Search doesn't include the documentation produced by the other repositories

The search functionality relies on the [search_index.json](https://github.com/apache/incubator-pekko-site/blob/main/content/search/search_index.json) file for indexing the content. Since the site is separate from the paradox docs of each repo, The `search_index.json` of the hompage does have the whole content indexed.

As result, the search only works when you use docs in the right area - the search data is built by all our different doc builds and each repo has its own doc build

https://pekko.apache.org/docs/pekko/current/ has the docs for https://github.com/apache/incubator-pekko
https://pekko.apache.org/docs/pekko-http/current/ has the docs for https://github.com/apache/incubator-pekko-http
So the search in https://pekko.apache.org/docs/pekko/current/ will support searching for actor and stream, etc.

See also: https://github.com/apache/incubator-pekko-site/issues/74

Start with content/search/search_index.json and compare how the homepage and the linked repository documentation builds produce their search data. Review issue 74 for related context. Done means the homepage search includes documentation from the other repositories, not only content in the local site area.

Written by the indexing model from the issue text.