[doc] Codify a rule for naming in cpp
[doc] Codify a rule for naming in cpp

 This rule a sintese of the Google C++ style guide for naming.

Signed-off-by: Douglas Reis <df95399f7253aef5711aa5e1f53e8824981313ab@lowrisc.org>

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