docs: Updated support library overview

Per feedback from the support library team, updated the support
library overview page to better explain what the libraries are
and what they're for. This is the beginning of a big overhaul of the
support library docs.

The "overview" section has been rewritten and expanded in this CL.
The "revisions" section hasn't been touched, except that the heading
was changed to "Support Library Revision History". (I'd like to
move "Revision History" to a separate page, but I'll do it in a
follow-on CL.)

See first comment for doc stage location.

bug: 27274455
Change-Id: I12ce21c948421e1bd0be5d3151845f900750e7b1