Initial ReadTheDocs Setup

[lukelowry]

Description

Adds initial Read the Docs support for GridKit documentation.

The live proof of concept is here: https://gridkit.readthedocs.io/en/latest/

I tested this on a fork because I do not have permissions to connect RTD to this repository. I will release the RTD project/domain name once this is merged.

Proposed changes

• Add .readthedocs.yaml, Sphinx config, and docs requirements.
• Generate RTD pages from existing README files instead of duplicating them.
• Structure RTD into Installation, Applications, Models, Examples, API Reference, and Development.

Checklist

• All tests pass for the focused coverage listed below.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra. N/A, docs-only.
• The new code follows GridKit™ style guidelines.
• N/A There are unit tests for the new code. n/a docs-only.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR.

Further comments

Setting up on the RTD side takes less than 30 seconds.

Typically, the model documentation READMEs would be in docs/ and readthedocs expects that so we have a temporary script that pulls those into docs/ as a pre-build task for RTD

[shakedregev]

It's not clear to me where this is pulling from. Why is the Heaviside function defined as $\sigma(x)$? This is confusing.

[lukelowry]

It is the logistic function. This symbol is used for it, so I use it also for the scale at infinity (which is also the Heaviside function).