[WIP] irradiance.py updates: glossary term links and units

[RDaxini]

• Relates to: Standarizing pvlib typesetting of units - standarization for common variables? #2205, [DOC]: params & units guidelines in contributing page #2248, Glossary page updates tracking #2284, GSoC feedback on the pvlib contribution process #2081
• 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.

Updating the units to superscript and linking key terms to their glossary page definitions.
Follow up PR(s): add some of these key terms into the glossary, enhance existing glossary term definitions (units and explanation)
Note: we can now view definition tooltips by hovering the cursor over the linked glossary term (context: #2290)

[RDaxini]

I could modify the scope of this PR if we want to see some of the changes implemented in 11.2. I don't think these changes are urgent though so I'm also happy to keep working on it and merge the completed version in 11.3. Not sure what reviewers would prefer though--- on second thoughts many small revisions covering the entire irradiance.py might be a pain to review. Thoughts @kandersolar @AdamRJensen @cwhanse ?