Make search_index a JSON file

[aaruni96]

Right now, search_index is a javascript file, who's only purpose is to assign the index to a variable called "documenterSearchIndex". Instead, it could be a valid JSON file, and we could load it into the variable from within assets/html/js/search.js. The end result would behave the same way, with the added benefit that projects using Documenter could manipulate the index json to permanently change results, instead of trying to patch the the final documenter.js to add more filters to do at search time.

I can submit a PR for this in a bit.