SubmittingPatches: move the patch-flow section earlier

SubmittingPatches: move the patch-flow section earlier

Before discussing the small details of how the patch gets sent, we'd
want to give people a larger picture first to set the expectation
straight.  The existing patch-flow section covers materials that are
suitable for that purpose, so move it to the beginning of the
document.  We'll update the contents of the section to clarify what
goal the patch submitter is working towards in the next step, which
will make it easier to understand the reason behind the individual
rules presented in latter parts of the document.

This step only moves two sections (patch-flow and patch-status)
without changing their contents, except that their section levels
are demoted from Level 1 to Level 2 to fit better in the document
structure at their new place.

Signed-off-by: Junio C Hamano <gitster@pobox.com>