Better support for local search plugin authors

[slorber]

In #7751 was discussed some challenges in implementing a reliable local search plugin.

Usually, local search plugin authors create the index in the postBuild lifecycle by scraping the HTML files.

That process is not always easy to get right in a non-hacky way, and Docusaurus should make it easier

If you are a local search plugin author, this discussion is a good place to discuss some challenges you are facing!