Document (new) gerrit review process and git/bitbake workflows for sandboxes

Sandboxes work and we have doc in the wiki

Topic still applies, but I can't see how I can help. Re-assigning (for closing?)

note that the reference http://docs.zephyrproject.org/1.2.0/collaboration/code/gerrit_practices.html (see above) is invalid

I just added the description of issues in submitting sandboxes in review.

Further reading:

• https://docs.openstack.org/infra/manual/sandbox.html

• http://docs.zephyrproject.org/1.2.0/collaboration/code/gerrit_practices.html

• (please add your links)