docs: stable-kernel-rules: improve structure by changing headlines

docs: stable-kernel-rules: improve structure by changing headlines

* replace a needless sub-heading with a short intro sentence
* make "Following the submission" a proper sub-section with a headline
  without changing the text of the section

CC: Greg KH <gregkh@linuxfoundation.org>
CC: Sasha Levin <sashal@kernel.org>
CC: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/r/0737676f951050b2ec39e1662ffea37d77ef0bec.1691219455.git.linux@leemhuis.info
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>