WIP: PSF Photometry API documentation for established functionality

[Onoddil]

This PR is part of the process for establishing the block API for PSF photometry. The documentation in this PR is unlikely to change as the four blocks here -- background estimation, source finder, source grouper, and PSF model -- are either independently used within photutils and thus well tested, matured and established pieces of code, or are relatively straightforward and thus leave little room for API changes. Please see #766 for an overview of the fitting process.

The documentation for background_estimator and finder should be very stable to changes, as these are the external function calls to routines used significantly throughout photutils. For these the primary issues are wording and formatting.

group_maker however is slightly less stable, and thus in addition to checking for clarity and formatting this block description must succinctly and clearly explain the dependency of the group maker on the scene maker (developed in parallel with the extension of PSF fitting to non-point sources, in which "groups" can include the merging of several point sources into an extended object and vice versa).

psf_model is a relatively minor documentation block, as it is primarily an external astropy class for which the PSF model must be a subclass, with specific input parameters. Thus this documentation must also explain this dependency.

The issues to be discussed in this PR are therefore:

• Background estimator
  • Formatting
  • Wording
• Finder
  • Formatting
  • Wording
• Group Maker
  • Formatting
  • Wording
  • Scene maker dependency
• PSF Model
  • Formatting
  • Wording
  • Fittable2DModel dependency

Please provide comments on any clarifications, inconsistencies, formatting changes or documentation flow changes here.