exclude nested namespace from doxygennamespace

[pwm1234]

I would like to be able to use the doxygennamespace directive for immediate contents of a namespace and exclude nested namespaces. I envision option choices along the lines of

• :no-nested: to skip all nested namespaces, or
• :exclude-nested: foo to skip the nested namespace foo (similar to the :members: option list for classes.

As with the other related issue I just submitted issue #531, if this is a reasonable request and someone can outline an approach to a solution, I am willing to try to implement a change and submit a PR if I can get something working.

[vermeeren]

Probably relevant: https://github.com/michaeljones/breathe/issues/531#issuecomment-628766147