docs: sphinx/kernel_abi: reduce buffer usage for ABI messages

docs: sphinx/kernel_abi: reduce buffer usage for ABI messages

Instead of producing a big message with all ABI contents and then
parse as a whole, simplify the code by handling each ABI symbol
in separate. As an additional benefit, there's no need to place
file/line nubers inlined at the data and use a regex to convert
them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/15be22955e3c6df49d7256c8fd24f62b397ad0ff.1739182025.git.mchehab+huawei@kernel.org