doc/reference: include PluginManager in PytestPluginManager and remove it
doc/reference: include PluginManager in PytestPluginManager and remove it

Before, `PluginManager` was a copy of the pluggy doc, and
`PytestPluginManager` was documented separately.

pytest users only really need to know about `PytestPluginManager`, so
instead of splitting have the `PytestPluginManager` documentation
include all of `PluginManager` members and remove `PluginManager` from
the reference (it is still shown as the base class).

RonnyPfannschmidt/pytest,The-Compiler/pytest,nicoddemus/pytest,markshao/pytest,nicoddemus/pytest,The-Compiler/pytest,Akasurde/pytest,pytest-dev/pytest