docs: sysctl/fs: re-order, prettify

This brings the text markup in line with sysctl/abi and
sysctl/kernel:

* the entries are ordered alphabetically
* the table of contents is automatically generated
* markup is used as appropriate for constants etc.

The content isn't fully up-to-date but the obsolete entries are gone,
so remove the kernel version mention.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20220930102937.135841-6-steve@sk2.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>