Add context documentation

[RobbeSneyders]

Contributes towards #1531

Fixes #937

[coveralls]

coverage: 94.422%. remained the same when pulling 0923872 on feature/context-docs into 655ea43 on main.

[Ruwann]

Thanks, LGTM!

Although there is still the limitation in the security functions as the operation (specification) is not available there (which is mentioned in one of the comments)

[RobbeSneyders]

Yes true. I'll add it to the security documentation.

We could make it available btw. The SecurityOperation that calls the security function has access to the Operation specification, as well as the other information we set as context variables.

We could either pass it into the security function directly, or set (and unset?) the context variables in each middleware, so code executed by the middleware has access to the context at that stage of the stack. Eg. custom validators would be able to use it as well. Let's discuss further in #1750