Glossary page updates tracking

[RDaxini]

Following on from discussions in #2234 and elsewhere, I thought it would help to create a single separate issue to track and discuss suggested further updates to that page.

• Create page (Change Variables and Symbols page into a Glossary of terms #2234)
• Add units (e.g. irradiance.py updates: glossary term links and units  #2311)
• "irradiation" -> "irradiance" (comment, Update irradiance component definitions in nomenclature.rst #2289)
• Adopt sphinx-hoverxref (comment, Implement hoverxref for floating tooltips #2290)
• Enhance definitions (refer to dni example)
• Add new definitions (e.g. add spectra and spectra_components to nomenclature page #2264)
• Address overlaps and inconsistencies (e.g. Standardise variable name for clearsky GHI and DNI, add to glossary #2272)

Supporting: #2234
Related / additional context: #1421

[RDaxini]

@kandersolar could we re-open this? still a few tasks left

[kandersolar]

Looks like it was auto-closed by merging @2290. That's caused by using magic words like "closes". If you want to mention an issue but not close it, you have to use vaguer words. I think "partially addresses" is safe... :P