doc: Update HACKING file
doc: Update HACKING file

- Add submission information

- Remove dependencies list. It does not add much value and it is
  difficult to maintain this list up-to-date whenever a new dependency
  gets added.

- Update information about building docs

freedomboxtwh/Plinth,freedomboxtwh/Plinth,kkampardi/Plinth,harry-7/Plinth,jvalleroy/plinth-debian,jvalleroy/plinth-debian,kkampardi/Plinth,freedomboxtwh/Plinth,vignanl/Plinth,jvalleroy/plinth-debian,harry-7/Plinth,jvalleroy/plinth-debian,kkampardi/Plinth,harry-7/Plinth,jvalleroy/plinth-debian,vignanl/Plinth,freedomboxtwh/Plinth,kkampardi/Plinth,vignanl/Plinth,harry-7/Plinth,freedomboxtwh/Plinth,harry-7/Plinth,vignanl/Plinth,vignanl/Plinth,kkampardi/Plinth