Rename docs/specs/fw_cfg.txt to .rst

This is a separate commit in order to make reviewing the next one easier.

Signed-off-by: Simon Sapin <simon.sapin@exyr.org>
Message-Id: <20220625161455.1232954-1-simon.sapin@exyr.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

diff --git a/docs/specs/fw_cfg.txt b/docs/specs/fw_cfg.rst
similarity index 100%
rename from docs/specs/fw_cfg.txt
rename to docs/specs/fw_cfg.rst