doc: add references to builds mailing list

builds@lists.zephyrproject.org archives the CI build results for every
and nightly build.  Add references to this in the contribution
guidelines and introduction (pulled from README).

Also added a reference to the "Asking for Help" doc (there was a note to
add this once that doc was merged).

Fixes: #15652

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>