This change allows users to specify recursive options under `members`:```::: some.module    options:      members:      - name: SomeName        options:          heading: Some Verbose Name          members:          - ...```This change also brings a refactor in how we combine default, global andlocal configuration. Instead of keeping track of dictionaries to mergethem accordingly, we now chain dataclasses together, using a special`UNSET` value to tell whether an option was set or not. Checking againstthis `UNSET` value lets the chained dataclass know whether it shouldtry to fetch the value from previous (left/up) dataclasses.With the following example chain:```{ some_option: UNSET } -> { some_option: False } -> { some_option: UNSET}```...trying to get `some_option` from the chain would start with theright-most dataclass, where `some_option` is `UNSET`. Then it wouldcontinue on the left, and return `False` since it's not `UNSET`. Theleft-most dataclass would not be checked since the value was alreadyreturned.This change lets us simplify templates, as we don't have to check ifwe're rendering the root object anymore: the configuration options arechained in a way that prevent options that are only relevant to the rootobject to be propagated further down to members.Issue-658:mkdocstrings/mkdocstrings#658