- Move per-client samples/links to client/README.md. This dramatically reduces
the size of the main repo's readme, making it a more effective landing point.
It also de-emphasizes the README-provided samples, which are less maintained
and less canonical than the cloud.google.com and godoc samples.
- Several link clean ups.
- Remove defunct NEWS section.
- Remove TOC.
- Remove reference to defunct googlegroups.

Change-Id: Ib97ecd825abb1616236be044bac975cb8ee50a72
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/41590


Reviewed-by: kokoro <[email protected]>
Reviewed-by: Tyler Bui-Palsulich <[email protected]>