tools: docs: parse_data_structs.py: add methods to return output

tools: docs: parse_data_structs.py: add methods to return output

When running it from command line, we want to write an output
file, but when used as a class, one may just want the output
content returned as a string.

Split write_output() on two methods to allow both usecases.

Also add an extra method to produce a TOC.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/c98bdec3380aad54178baf2751a2f1fcd128576b.1755872208.git.mchehab+huawei@kernel.org