docs: conf.py: use dedent and r-strings for LaTeX macros

docs: conf.py: use dedent and r-strings for LaTeX macros

Instead of adding extra weird indentation at the tex
file, use dedent(). While here, also use r-strings, to make
easier to make its content identical to the .tex output.

While here, also merge "preamble" that was added on two
separate parts of the code (in the past, there were some
version-specific checks).

No functional changes, just cosmetic ones.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/0d245fbd872ab3ec21bd8fe78b01340ba77ce365.1755763127.git.mchehab+huawei@kernel.org