docs: rewrite admin-guide/sysctl/abi.rst

docs: rewrite admin-guide/sysctl/abi.rst

Following the structure used in sysctl/kernel.rst, this updates
abi.rst to use ReStructured Text more fully and updates the entries to
match current kernels:

  * the list of files is now the table of contents;
  * links are used to point to other documentation and other sections;
  * all the existing entries are no longer present, so this removes
    them;
  * document vsyscall32.

Mentions of the kernel version are dropped. Since the document is
entirely rewritten, I've replaced the copyright statement.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20200911190152.29730-1-steve@sk2.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>