export/html: RST->HTML rendering using docutils is a performance bottleneck

[stanislaw]

1. It can be measured what takes more time: parsing RST tree or actually rendering HTML
2. Simplified RST parser and rendered can be written and their performance can be compared with that of docutils API.

python -m cProfile -s cumulative strictdoc/cli/main.py export --no-parallelization docs/ > report.txt

https://docs.python.org/3/library/profile.html#instant-user-s-manual