ci/test-documentation: work around SyntaxWarning in Python 3.12

ci/test-documentation: work around SyntaxWarning in Python 3.12

In Python 3.6, unrecognized escape sequences in regular expressions
started to produce a DeprecationWarning [1]. In Python 3.12, this was
upgraded to a SyntaxWarning and will eventually be raised even further
to a SyntaxError. We indirectly hit such unrecognized escape sequences
via Asciidoc, which results in a bunch of warnings:

    $ asciidoc -o /dev/null git-cat-file.txt
    <unknown>:1: SyntaxWarning: invalid escape sequence '\S'
    <unknown>:1: SyntaxWarning: invalid escape sequence '\S'

This in turn causes our "ci/test-documentation.sh" script to fail, as it
checks that stderr of `make doc` is empty.

These escape sequences seem to be part of Asciidoc itself. In the long
term, we should probably consider dropping support for Asciidoc in favor
of Asciidoctor. Upstream also considers itself to be legacy software and
recommends to move away from it [2]:

    It is suggested that unless you specifically require the AsciiDoc.py
    toolchain, you should find a processor that handles the modern
    AsciiDoc syntax.

For now though, let's expand its lifetime a little bit more by filtering
out these new warnings. We should probably reconsider once the warnings
are upgraded to errors by Python.

[1]: https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals
[2]: https://github.com/asciidoc-py/asciidoc-py/blob/6d9f76cff0dc3b7ca21bdd570200f8518464d99b/README.md#asciidocpy

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>