As the build environment documentation is duplicated across each of the
Keyman platform projects, it has become somewhat convoluted and
inconsistent. This PR moves all the mac-based and shared documentation
into docs/build, with placeholders for Windows and Linux documentation
to be moved in the next PR.
The idea is that a developer can configure their box to build any of the
available projects for that platform (web and android build on all three
main dev platforms).
Keeping the build documentation in one place makes it much simpler to
keep it consistent and up-to-date as well, as well as keeping us aware
of potential dependency conflicts between projects.
The icing on the cake is the resources/devbox/macos/macos.sh script,
which I have tested and takes a bare metal mac box through to a full
Keyman development environment (except XCode, there's always something!)
