process.rst: Modernize the "Workflow of a Custodian" section

process.rst: Modernize the "Workflow of a Custodian" section

The "Workflow of a Custodian" section on the wiki had not been changed
in quite some time to reflect how the process has been functioning for
some time.  First, update some links to point to modern and current
sources of information.

Second, and more overarching, reword much of the section.  This expands
on the expectations of both custodians and developers when it comes to
rebasing patches.  Rework the final points to be clearer that Custodians
are expected to do their best to test the changes and ask for help when
needed, as well as that pull requests are expected in a timely manner.

Cc: Claudius Heine <ch@denx.de>
Cc: Martin Bonner <martingreybeard@gmail.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>