Update icon documentation, add a tool, and commit .car file

Update the documentation on how to compile an asset catalog, and add a
tool to build the final .car file.

This also commits a compiled version of the asset catalog (using the
tool) that already exists in the repo.

Bug: 425894794
Change-Id: I394c925f33d2a3ee8a766777bba06005a5835b85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6701309
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1483911}