TYP/DOC: add HTMLFlavors type to read_html and related
Following up on what was discussed in https://github.com/pandas-dev/pandas-stubs/pull/772, accepted html flavors are now defined in their own type instead of being a str or a generic str sequence.

Refs:
- https://github.com/pandas-dev/pandas-stubs/pull/772#discussion_r1322037456
- https://github.com/pandas-dev/pandas/issues/55059
- https://github.com/pandas-dev/pandas/pull/55076
Thanks @matheusfelipeog 