Reorganize the documentation

[ChrisRackauckas]

This comes after some hefty discussions with users, customers, etc. along with surveys, analytics, and all of that. We'll be rolling out a slightly perturbed style throughout all of the packages, starting with the biggies, being ModelingToolkit and DifferentialEquations.

This comes from some discussions with folks who found the documentation of the following styles to be quite good:

• https://pandas.pydata.org/docs/index.html
• https://matplotlib.org/stable/index.html
• https://www.mathworks.com/help/matlab/ordinary-differential-equations.html?s_tid=CRUX_topnav

One noticeable aspect is the separation of tutorials from examples. While these all do something slightly different of course, the general mantra is that tutorials are more beginner explanations of core functionality while examples are just "cool shit" you want to show, which maybe has less text. The latter is mostly useful for showing people what you can do, or giving people starter code. The former is more about a human-level explanation. We had been traditionally mixing the two, but it's good to split them so they can serve their respective functions better.

Additionally, for each package we are designating one tutorial to be on top as the "Getting Started" tutorial. The SciML docs as a whole have a "Getting Started" section which we use as a landing page that orients people to the whole ecosystem (https://docs.sciml.ai/Overview/stable/). It's in-depth, multipage, etc. and similar in style to the Pandas or SciPy ones. This then leaves room for the "Getting Started" section of a package to simply be about the package itself. It should have a section at the end that contextualizes, like is seen right now in DifferentialEquations.jl (https://docs.sciml.ai/DiffEqDocs/v7.6/tutorials/ode_example/#Additional-Features-and-Analysis-Tools), but its real focus is "99% of people who use this package should know at least these things". Our Google Analytics shows that >90% of all users read the first tutorial (and about 20% only read the first tutorial in DifferentialEquations, but seem to keep coming back to it!). Thus we basically want a page that will serve as something we "know" or can assume every reader has read. This is why it's then elevated from being the first tutorial to being a separate highlighted "Getting Started" page (which should then also include installation instructions and links to other tutorials / videos).

For ModelingToolkit and DifferentialEquations, and many of our other packages, the first tutorial has already been written as a "Getting Started" type of tutorial, while the other tutorials then dig into specific features. So this PR, like the ones that will happen to other package, elevates the first tutorial to this "Getting Started" status.

That is not to say everything is completed. The matplotlib and MATLAB documentation examples show that associating a plot with each example and putting them in a tiled view is really nice and accessible. We cannot do that with Documenter.jl, so for now we will at least get the examples curated while we beg for some new way to distinguish them from the rest of the documentation. Additionally, I have taken the following notes for documentation improvements, which I'll put here but not actually do yet in this restructuring PR:

• Make as many of the comparisons and explanations into tables. This reduces text and improves readability
• Format examples using more than just headers. Use some bolding, italics. Plots in grids, etc.
• Move plots up, add plots to tutorials
• Make a table introducing all of the modules being used at the top
• Get Documenter.jl to change its default font size from being high school cartoon to something more professional
• Allow more white space in the pages. Space to digest is more important than keeping pages short
• Don't go off template: change the Documenter template because being uniform is more important than being better

[ChrisRackauckas]

Interesting comment: along with the modules at the top, also include which skills are demonstrated, with a link to the pages about those skills. So for example, a page will say that the skills demonstrated are ODE modeling, event handling and callbacks, and parallelism at the top, and then link to those 3 pages. That way at a glance you can know what will be in there, and know where to get more information.

The content pages could back link to the examples too. This would likely need some help from Documenter itself, but can be done by hand for now.

[ChrisRackauckas]

@YingboMa I think the example was hitting a structural_simplify bug: https://github.com/SciML/ModelingToolkit.jl/actions/runs/3669263610/jobs/6202966279#step:5:19 . Disabling it for now, but it would be good to make sure that's revived since it's in the docs