update to ubuntu 20.04 and fix typos

[dumprop]

Ported ubuntu bootstrap script to 20.04 version because 18.04 EOL soon I also slightly corrected the text in the documentation As it turned out, the public documentation was not updated for a some time because the _build folder is located in gitignore, so probably it should be removed (and then do a sphinx rebuild)? Then in close #690 #718