Docs: Refine the contribution process

- Remove the bullets of Phabricator which is rarely used now.
- Update the design proposal description. Clarify the process
  accordingly.
- Refer to Gerrit offical documents for gerrit operation details
  instead of making simplifed descriptions.

Change-Id: I8706e019f95c0e528ff34c2ada856d14bd98aeab
Signed-off-by: David Hu <david.hu@arm.com>
1 file changed