doc: sending_patches.rst: Incorporate the old "Patches" wiki content

Import as-is much of the old "Patches" wiki page to the current
sending_patches.rst file. This means we need to move patman to being
included in the higher level ToC and add a reference for "Custodians" in
the process document. A very minimal amount of content changing and
rewording is done here as part of the import, in order to make the
conversion easier.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/process.rst b/doc/develop/process.rst
index 388945c..6a18a81 100644
--- a/doc/develop/process.rst
+++ b/doc/develop/process.rst
@@ -108,6 +108,8 @@
   In U-Boot, ``"-rc1"`` will only be released after all (or at least most of
   the) patches that were submitted during the merge window have been applied.
 
+.. _custodians:
+
 Custodians
 ----------