Fix #291, #292 and add unit tests for deserializing Mkdocs/Hugo example configuration

[NiklasRosenstein]

Users that use Pydoc-Markdown as a library and implement their own Page subclass like so:

from pydoc_markdown.util.pages import Page

class MyPage(Page["MyPage"]):
  ...

Should update their code to read:

from pydoc_markdown.util.pages import GenericPage

class MyPage(GenericPage["MyPage"]):
  ...

This will avoid Mypy errors like this below:

error: "Page" expects no type arguments, but 1 given  [type-arg]

The type stays compatible with the old semantics at runtime, so this is not a breaking change.

Introducing the GenericPage is the fix for #291.

292 is fixed by upgrading Databind to ^4.4.0.