Fix some documentation issues

[enra64]

Fixes the following entries from the pyOpenSci entry issue:

• (Could be improved) Add a link to CONTRIBUTING.rst for development installation.
• (Must be improved) jointly.helpers.get_segment_data: documentation is missing
• (Must be improved) jointly.helpers.get_stretch_factor: only looking at the documentation, it is not clear for me what should be passed as a segment and timeshift. Doc could be improved or an example might be necessary
• (Must be improved): https://jointly.readthedocs.io/en/latest/reference.html#module-jointly.helpers Helpers section description starts with "Contains plotting helpers". However, no plotting helpers documentation is found here. Given that functions in helpers_plotting.py have a docstring I would add them to the documentation
• (Must be improved) jointly.segment_selectoris empty
• Some projects prefer developers to submit an issue with the new feature suggestion before pull requests with new implementations. If that is your case I would suggest to add an indication.
• History section is good! I find it more useful when the pull request link is found next to the new features/bugs corrections when applicable. Might be a good idea to incorporate that to the contributing guidelines
• Problems found to run the tests locally as described in contributing section; should close ModuleNotFoundError when trying to run pytest #10, can't test as I don't have conda
• The internal function calculate_magnitude has some behaviour that might be unexpected based on its name. This behaviour has been added to the documentation. Closes jointly.calculate_magnitude returns 0 for NaN inputs #11.