Miscellaneous issues in Docs

[kyleaoman]

• In this docs page: https://petrofit.readthedocs.io/en/stable/photometry.html there is a bullet list near "four plots are displayed" that does not seem to have rendered correctly (though it looks fine if I open the notebook in the repository).
• In https://petrofit.readthedocs.io/en/stable/petrosian.html there is a heading "Adopting ``epsilon=0.5`` & ``epsilon_fraction=0.5``" where backticks have not rendered as fixed-width text. Similarly in https://petrofit.readthedocs.io/en/stable/fitting.html there is "Note that ``PSFModel`` is deprecated and replaced by ``PSFConvolvedModel2D``"
• In general it would be useful to link references to functions (let's say get_default_sersic_bounds in https://petrofit.readthedocs.io/en/stable/fitting.html to be concrete) to the corresponding entries in the API documentation using sphinx's crossreferencing features.
• One step further, using intersphinx it's also possible to link to external docs pages. As one illustrative example, in https://petrofit.readthedocs.io/en/stable/fitting.html, there is "PetroFit uses the astropy modeling sub-module". It seems like it would make sense for "astropy" and/or "modeling" to link to the relevant documentation from astropy. I'm not sure if/how intersphinx works with docs generated from notebooks...
• At the bottom of the https://petrofit.readthedocs.io/en/stable/multi_object.html# page, the "next" link leads to https://petrofit.readthedocs.io/en/stable/api_reference.html instead of https://petrofit.readthedocs.io/en/stable/multi_object.html#Simultaneous-Fitting (ah I see now that this is a section within the same page... should that header perhaps be one level deeper?)
• Should https://petrofit.readthedocs.io/en/stable/release_procedure.html mention creating a zenodo archive?
• In https://petrofit.readthedocs.io/en/stable/multi_object.html there is "as described in the Estimating Sersic Parameters section." that seems like it should be cross-referenced.
• Some function docstrings are missing documentation of return values, such as petrofit.modeling.petrosian_profile (numpydoc could be used to find all such omissions).
• https://petrofit.readthedocs.io/en/stable/api/petrofit.petrosian.Petrosian.html has some attributes with missing docstrings. https://petrofit.readthedocs.io/en/stable/api/petrofit.petrosian.PetrosianCorrection.html has some functions and attributes with missing docstrings.
• segmentation module has several functions missing parameter descriptions in docstrings.
• Several functions in utils module missing docstrings. If these are meant to be for internal use only, mark as private with leading underscore and ask sphinx to omit private members from documentation?
• https://petrofit.readthedocs.io/en/stable/api/petrofit.utils.plot_target.html has formatting error in notes section.
• I've just directly fixed a number of very minor typos etc in Misc fixes noticed during PyOpenSci review #220, this should be reviewed and merged.

This issue is part of my PyOpenSci review. For review pass/fail purposes, resolving it is optional.

[robelgeda]

descoping due to doc update to come