Added a lightweight "core" Makefile entry

`make core` now runs a lightweight doc build process that doesn't include the
"api" target. The "html" target now just references "core" and "api".

This patch is a candidate for backporting to 1.x.
Renamed htmlcore target, reordered api & htmlcore
Modified recipe so api is always executed first

Also renamed corehtml to html_noapi, and a dependency on clean to html_noapi.
Merge pull request #4208 from davclark/docs-makefile-speedup

Added a lightweight "htmlcore" Makefile entry