Docs: Remove .rst from docs for simpler build

[tpike3]

Use of .ipynb transitioned to .rst for the Sphinx build can result in desync from readthedocs build and.ipynb files and large diff files.

We need to update to using just .ipynb with Sphinx.

Discussed in #1361

[Corvince]

To continue the discussion here:

nbsphinx can can evaluate, i.e. run, notebooks once during doc generation, but does not allow interactive notebooks (this would require some backend). This still means we can provide source control friendly unevaluated .ipynb files and they will show just fine.

~~Re client side ABM modeling: I think agentscript is the most popular JS ABM library, at least it was some years ago.~~

Edit: sorry I was misremembering. I just looked it up again and Agentscript is not very popular.

[rht]

#1694 is an attempt to fix this.

Regarding with having an executable tutorial, I think it would work just fine if we provide a Binder/Colab link to the .ipynb files, described in the tutorial themselves.

[tpike3]

.rst files have been removed