Api docs

[quaquel]

This PR reorganizes the API documentation.

It is a follow-up of #2294. The current structure of the API overview in readthedocs has become more messy partly due to explicitly using and parsing Google docstrings. Moreover, the API docs reflect a slightly outdated view of the core classes in MESA. This PR reorganizes the API docs in such a way that it is just a convenient way to get to the docs of all relevant classes. Tutorials and other docs are available to highlight how to use MESA. The API docs should be a straightforward reference manual and be easy to navigate.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 -0.5% [-1.3%, +0.3%]	🔵 -0.8% [-1.0%, -0.6%]
BoltzmannWealth	large	🔵 +31.0% [+1.6%, +89.0%]	🟢 -7.1% [-11.5%, -3.4%]
Schelling	small	🔵 -0.6% [-0.9%, -0.2%]	🔵 -1.1% [-1.4%, -0.8%]
Schelling	large	🔵 -0.4% [-1.5%, +0.7%]	🟢 -5.5% [-6.9%, -4.0%]
WolfSheep	small	🔵 -0.7% [-2.0%, +0.5%]	🔵 -0.7% [-1.1%, -0.4%]
WolfSheep	large	🔵 -2.7% [-4.6%, -1.1%]	🟢 -7.3% [-9.0%, -5.8%]
BoidFlockers	small	🔵 +1.6% [+1.0%, +2.1%]	🔵 +0.7% [-0.0%, +1.3%]
BoidFlockers	large	🔵 +2.3% [+1.7%, +2.8%]	🔵 +0.1% [-0.3%, +0.6%]

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🟢 -5.7% [-6.4%, -5.1%]	🔵 -1.0% [-1.2%, -0.9%]
BoltzmannWealth	large	🔵 -8.2% [-22.8%, -1.0%]	🟢 -9.6% [-10.9%, -8.2%]
Schelling	small	🔵 -1.2% [-1.5%, -0.9%]	🔵 -1.5% [-1.8%, -1.2%]
Schelling	large	🔵 +0.4% [-0.1%, +0.9%]	🔵 +3.7% [+2.6%, +4.8%]
WolfSheep	small	🔵 -1.6% [-2.8%, -0.4%]	🔵 -0.3% [-0.6%, +0.1%]
WolfSheep	large	🔵 -1.7% [-3.6%, -0.3%]	🔵 -2.2% [-4.3%, -0.5%]
BoidFlockers	small	🔵 +3.1% [+2.6%, +3.5%]	🔵 -1.2% [-2.0%, -0.5%]
BoidFlockers	large	🔵 +2.8% [+2.5%, +3.0%]	🔵 -2.0% [-2.8%, -1.2%]

[EwoutH]

Diff looks good, new rendered docs also. Good to have the AgentSet in the API docs!

[EwoutH]

With hindsight, I liked the "Base Classes" concept. It separate the classes we expect users to subclass from other ones. Maybe we can structure it like this: