Convert test catalog generation to readthedocs.io (sphinx)

[pevik]

Original test catalog generation was written long time ago we started using sphinx on https://linux-test-project.readthedocs.io/.

Convert perl to produce generation via doc/conf.py. It will be page in https://linux-test-project.readthedocs.io/ instead of separate html/pdf.

After this whole asciidoc/asciidoctor detection in configure.ac, m4/ax_prog_perl_modules.m4, m4/ltp-docparse.m4 can be deleted.

NOTE: We want to keep LTP specific format for headers, e.g. [Description] or [Algorithm]. We don't want to convert them to header level 1 (e.g. not # Description) because we don't want to didn't want to have any heading levels exposed in the doc comments. Also bullet lists or enumerated lists looks the same in reStructuredText, thus doc in the tests does not need to be touched.