[docs] Add Build Process documentation (#691)

Some PRs add features which should ideally be documented as part of
their commit, e.g. a35e52a, which adds
`$(AndroidPreferNativeLibrariesWithDebugSymbols)`, and 406df8a,
which adds `$(AndroidBuildApplicationPackage)`.

However, documentation is maintained separately in the
xamarin/documentation repo, which makes it very easy to overlook
or forget to write documentation for new features.

Solve this problem by copying xamarin/documentation's
`guides/android/under_the_hood/build_process/index.md` into
`Documentation/build_process.md`. This allows future feature PRs to
update `build_process.md` and provide the initial doc effort.

We will need to manually synchronize the internal and public versions
of this file on an ongoing basis.

The online canonical version is at:

<https://developer.xamarin.com/guides/android/under_the_hood/build_process/>

Author: dellis1972