Don't assume that there will be an extra `context` kwarg when using the
bookmark serializer. We use it this way in the current code but that's
specific to us and not comon to all serializers. There are a lot of API
documentation tools that automate introspecting serializers but they
won't know that they have to send in a `context` to the serializer.
To make this serializer behave more like other serializers without
changing the behavior, we just need to check that the `context` value is
defined before we dig into it. In the case that there is no `context`
we just treat it the same as if there is no `request` in the `context`.
The API documentation decorators do not have to leak which solution we
use to generate the docs. Here, and as discussed in PR #21820, we rename
the `openapi` module to `apidocs`, and we make sure that this module
includes all the right functions to document API Views without referring
to Open API.
