Update realtime compiler to generate search indexes on demand

[caendesilva]

Previously we required a site build to generate the documentation search index, leaving the realtime compiler to return a 404. This update automatically generates the index on each request.

I decided against caching because of how fast the generator is. The request time (which is asynchronous) is increased to ~160ms instead of ~30ms for the entire HydePHP documentation. Considering that this happens in the background, I find this an entirely acceptable tradeoff.

Fixes #1404

[codecov]

Codecov Report

Merging #1405 (4ba0cfa) into master (a42101d) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master    #1405     +/-   ##
===========================================
  Coverage     99.97%   99.97%             
- Complexity     1705     3410   +1705     
===========================================
  Files           180      360    +180     
  Lines          4619     9238   +4619     
===========================================
+ Hits           4618     9236   +4618     
- Misses            1        2      +1     

see 180 files with indirect coverage changes