Improve various docstrings

[misi9170]

This PR makes a handful of updates to docstrings throughout FLORIS to respond to minor issues that have been raised.

• Adds details on the serial refine yaw optimizer Documentation: Better documentation of yaw_optimizer_sr.py #1021
• Adds a simple docstring for FlorisModel.get_defaults() FlorisModel.get_defaults() missing docstring #1161
• Updates references to floris.simulation (now floris.core) Documentation still refers to floris.simulation #1105 . In this case, I opted to remove a description of the floris input file structure and referred readers to the docs and the default_inputs.yaml for an example.
• Update variable name from r to r_squared in Gaussian models (see A question about the setting of 'n' in the gaussian_function(C, r, n, sigma). #1102).
• Add references to Katic et al. (1987) for sum of squares combination model.
• Updates the description docstring in examples/examples_turbine/001_reference_turbines.py, which I forgot to update in Add IEA 22MW reference wind turbine to turbine library #1146 .
• Fixes hyperlink formatting in READEME

[rafmudaf]

Hey @misi9170 I have a few things to add along these lines, if you don't mind waiting a few days for me to submit a PR to your branch

[misi9170]

@rafmudaf please do, no rush. Fine also for you to simply commit and push into my branch if you'd like.

[misi9170]

Converting to draft while I'm still planning to address a couple of things:

• Secondary items mentioned in Documentation still refers to floris.simulation #1105
• Details on the value of initial in the Crespo-Hernández model Feedback and suggestions for the added-turbulence module of Floris v-3.5 #773