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 <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/ABI/README

···
-This directory attempts to document the ABI between the Linux kernel and
+This part of the documentation inside Documentation/ABI directory
+attempts to document the ABI between the Linux kernel and
 userspace, and the relative stability of these interfaces.  Due to the
 everchanging nature of Linux, and the differing maturity levels, these
 interfaces should be used by userspace programs in different ways.

Documentation/admin-guide/abi-readme-file.rst

···
-.. SPDX-License-Identifier: GPL-2.0
-
-ABI README
-==========
-
-.. kernel-abi:: README

Documentation/admin-guide/abi.rst

···
 Linux ABI description
 =====================
 
+.. kernel-abi:: README
+
 ABI symbols
 -----------
 
···
 .. toctree::
    :maxdepth: 2
 
-   abi-readme-file
    abi-stable-files
    abi-testing-files
    abi-obsolete-files

scripts/lib/abi/abi_parser.py

···
     def parse_readme(self, nametag, fname):
         """Parse ABI README file"""
 
-        nametag["what"] = ["ABI file contents"]
+        nametag["what"] = ["Introduction"]
         nametag["path"] = "README"
         with open(fname, "r", encoding="utf8", errors="backslashreplace") as fp:
             for line in fp: