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.

• [x] Create page (#2234)
• [ ] Add units (e.g. #2311)
• [x] "irradiation" -> "irradiance" (comment, #2289)
• [x] Adopt sphinx-hoverxref (comment, #2290)
• [ ] Enhance definitions (refer to dni example)
• [x] Add new definitions (e.g. #2264)
• [x] Address overlaps and inconsistencies (e.g. #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