Suggestion: reorganise user guide folder with subfolders

[RDaxini]

Is your feature request related to a problem? Please describe.
The user guide folder is becoming pretty packed with a range of different files. More files are soon to be added, e.g. through #2353.

Describe the solution you'd like
Would it be helpful to organise the files using subfolders? These could align with the structure of the new readthedocs subsections (#2310), as one possibility.
I think subfolders would make navigating the folder easier now, and even more so once more pages are added (#2329).

Describe alternatives you've considered
No subfolders...

[cwhanse]

We're having this discussion at my lab. In my view, the current User Guide would be better separated into three subheadings:

1 Getting Started - the Introductory tutorial is the only content we have that I would put here
2 Modeling Guide - steps through the modeling process giving a bit of theory for each step, and a guide to implementation in pvlib
3 Technical Reference - material that is supplementary to the modeling guide, such as the details about the single diode equation and its solution

[RDaxini]

@cwhanse just to clarify, are you talking about subsections for the section navigation pane on the user-facing readthedocs site, or sorting the .rst files, which are inside the user guide folder, into subfolders of those names?

[cwhanse]

I was thinking about the user-facing navigation panes.

[RDaxini]

Ah okay, since I thought that matter was settled in #2310, in this issue I was thinking about the file organisation. I'm open to reworking the user-facing subsections though. Is it possible to re-open a merged PR?

[cwhanse]

Let's not go back on merged PRs. What I put up here as an idea, is a subdivision of the Advanced usage section from #2310, into Modeling Guide and Technical Reference.

Sorry I confused your proposal (sort the files) with the user-facing menus.