Cleans up README and simplifies build steps

1. Remove leftover mentions of `make` in the context of building ORCA
1. Because ninja is parallel by default, remove mentions of how to
   parallelize the build

* Simplify build steps in README

Noticeably, we no longer require the two most hated steps:
cd-after-mkdir-build. Instead `cmake` will directly mkdir the build
directly if it doesn't exist.

* [ci skip]
This fixes #248