Improve docs extension for examples (#7900)
We recently added automatic download links for examples in our documentation, however they assumed everything to live inside the main cvc5 repo while, e.g., the examples for the z3py compat API do not. While #7896 provided a very simple fix, it mixes different concerns (rendering of tabs vs. locating the input files, though they should coincide right now).
This PR takes a more fundamental approach by extending the current pattern mechanism to allow for explicit url specification.
Closes #7896.