[doc/ug] Github commit & merge guide
[doc/ug] Github commit & merge guide

Now the repo only supports `Rebase and Merge` option, the doc is revised
to explain how to pack commits into a commit. Also, added guideline of
commit message while squashing.

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan