Enable prebuild indexing

[albert-schilling]

Is it conceivable to build the search index independently of the build step? E.g. only based on the markdown files in the docs folder?

We have the issue, that we are using client-side authentication in our Docusaurus build and thus, had to use a work around (as described here: #210 (comment) ) to solve this.

It would be great if you could choose between using the build folder or let's say the original docs folder to create the search index.