gh-35939: Support for exponential and gamma distribution
I think that the exponential and gamma distributions are essential.
Instead of filing an issue I tried to edit the code. Please have a look
at the changes.
### :books: Description
Support for exponential and gamma distribution from GSL.
### :memo: Checklist
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- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/35939
Reported by: Gerald Teschl
Reviewer(s): Matthias Köppe
