The script is intended as a tool for doing every sort of verifications amenable to Rustdoc's HTML output. For example, link checkers would go to this script. It already parses HTML into a document tree form (with a slight caveat), so future tests can make use of it.
As an example, relevant `rustdoc-*` run-make tests have been updated to use `htmldocck.py` and got their `verify.sh` removed. In the future they may go to a dedicated directory with htmldocck running by default. The detailed explanation of test scripts is provided as a docstring of htmldocck.
cc #19723
3bf41dafcf