Mark envsetup.sh vars as deprecated in makefiles

For the envsetup.sh variables that should not be used in makefiles
(since they're not explicitly set up, and won't be available on the
build servers), mark them as deprecated.

Rework our documentation to have a landing page, and create a "Changes"
section where we can record changes like these. At some point I may go
and backfill some recent work.

Test: build/soong/build_test.bash
Change-Id: I54b9294ddf270245afdb58d17150db8098584e8a