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
• "irradiation" -> "irradiance" (comment)
• Adopt sphinx-hoverxref (comment)
• 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