docs: submit-checklist: use subheadings

During review (see Link), Jani Nikula suggested to use proper subheadings
instead of using italics to indicate the different new top-level
categories in the checklist. Further the top heading should follow the
common scheme.

Use subheadings. Adjust to common heading adornment.

Link: https://lore.kernel.org/linux-doc/87o7c3mlwb.fsf@intel.com/


Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20240229030743.9125-3-lukas.bulwahn@gmail.com>