docs: ABI: correct QEMU fw_cfg spec path

docs: ABI: correct QEMU fw_cfg spec path

A few weeks ago, QEMU switched docs/specs/fw_cfg.txt to be
docs/specs/fw_cfg.rst, so update the reference in the kernel docs to
reflect this. Also add a link to the online documentation to make it
easier to find.

Cc: Simon Sapin <simon.sapin@exyr.org>
Cc: Gabriel Somlo <somlo@cmu.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://lore.kernel.org/r/20220720123131.341755-1-Jason@zx2c4.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>