Documentation/process: Add tip tree handbook

Add a document to the subsystem/maintainer handbook section, which explains
what the tip tree is, how it operates and what rules and expectations it
has.

  [ bp:

   - Add a SPDX identifier, work in most comments from the thread.
   - 9bf19b78 ("Documentation/submitting-patches: Document the SoB
     chain") is also in the main Documentation but I'm leaving the
     paragraph here because it has the proper structure - text talks about
     SoBs and referencing somewhere else would interrupt the flow.
   - Move backtraces in changelogs to main submitting-patches.rst.
   - "Patch version information" is explained to a great detail in
     submitting-patches.rst too.
   - Hyperlink resend reminders section.
  ]
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Paul E. McKenney <paulmck@linux.ibm.com>
Link: https://lkml.kernel.org/r/20181107171149.165693799@linutronix.de
Link: https://lore.kernel.org/r/20210913153942.15251-3-bp@alien8.deSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/maintainer-handbooks.rst

@@ -14,3 +14,5 @@ Contents:
 .. toctree::
    :numbered:
    :maxdepth: 2
+
+   maintainer-tip

Documentation/process/submitting-patches.rst

@@ -330,6 +330,7 @@ politely and address the problems they have pointed out.
 See Documentation/process/email-clients.rst for recommendations on email
 clients and mailing list etiquette.
 
+.. _resend_reminders:
 
 Don't get discouraged - or impatient
 ------------------------------------
@@ -715,6 +716,8 @@ patch::
 See more details on the proper patch format in the following
 references.
 
+.. _backtraces:
+
 Backtraces in commit mesages
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^