docs: ABI: move README contents to the top

docs: ABI: move README contents to the top

The ABI documentation looks a little bit better if it starts
with the contents of the README is placed at the beginning.

Move it to the beginning of the ABI chapter. While here, improve
the README text and change the title that will be shown at the
html/pdf output to be coherent with both ABI file contents and
with the generated documentation output.

Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/20250211055809.1898623-1-mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>