Correct `racking_model` default value, add `freestanding` option

[RDaxini]

• [X] Relates to #1942
• [X] I am familiar with the contributing guidelines
• [ ] ~Tests added~
• [ ] ~Updates entries in docs/sphinx/source/reference for API changes.~
• [x] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• [x] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• [x] Pull request is nearly complete and ready for detailed review.
• [x] Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

• ~Updated docstring value for racking_model from open_rack to None (comment)~
• Added freestanding option for racking_model
• Updated comment to remove specific reference to SAPM (since pvsyst or sapm could be used depending on the value of racking_model, if I have understood correctly)
• ~Added None as default value for module_height~

Other changes: Formatting, minor edits (backticks, units, remove unused package, etc.)

[RDaxini]

I have limited these changes to PVSystem and FixedMount only as these are the two classes mentioned in #1942 I could make further edits, e.g. more formatting, update/add default values for the same (and other) parameters in the other docstrings, etc. either in this PR if recommended, or in a follow up PR if this would become too cluttered (please advise)

[RDaxini]

Good to go?

[kandersolar]

One more thing -- can we copy the new racking_model description to SingleAxisTrackerMount as well?

[RDaxini]

One more thing -- can we copy the new racking_model description to SingleAxisTrackerMount as well?

I think I did originally somewhere in a past commit, but then after some discussion about scope and what was mentioned in the original issue, I reverted that... I'll go ahead and redo!

[RDaxini]

Thanks both for your input and reviewing !