Add FFD rule of thumb tip to mach aero tutorial and clean up some for…

[gawng]

Purpose

Some people ask how many FFDs is a good amount. This rule of thumb was not written anywhere.
Also formatting stuff

Expected time until merged

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

Checklist

• I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation

[hajdik]

I noticed function names weren't linked to the info in the docs so @gawng and I talked about adding them for this page so it's easier to find API information. I added all of the links except for write_wing_FFD_file() which I couldn't get to work.