Correct racking_model default value, add freestanding option

[RDaxini]

• Relates to Detailed explanation of racking_module and module_type in PVSystem docstring #1942
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• 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`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

Four primary changes:

• 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:
Basic formatting and other edits (e.g. adding units, swapping apostrophes for backticks), removed unused package

[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)

[cwhanse]

I agree with limiting the formatting fixes to the functions/classes touched by the primary purpose of the PR.