No change to logic, only docs. This upstreams the web page layout guide,
mostly autogenerated from the google sites page (tables were converted
by hand).

I made some minor edits to the text for readability, but the content is
mostly unchanged.

This omits the "experimental data" section on the original doc because
(1) this isn't very relevant anymore since the behavior has shipped and
we don't intend to reevaluate and (2) the data was internal-only.

Fixed: 1083508
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: Ic1ece33a5565986235af1dbb735d35966253fe5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393006Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804429}