Overhaul tutorial and `@docs` references and strings for core types

[Datseris]

Would you believe that this PR took me more than 8 hours?!?!?!

This PR overhauls the introduction to Agents.jl for new users, or returning veterans alike. It does:

• Completely re-write the tutorial from scratch. Now the main tutorial and the schelling model are merged into one. I took particular care to introduce the new v6 Agents.jl by stressing the existence of different models and the @multiagent macro.
• Adds a short copy-pasteable example in the tutorial, inspired by ModelingToolkit.jl docs that also starts with copy pasteable stuff.
• Improves the docstrings of core types for the models as well as the agent constructors.
• Changes where the docstrings are. Now they are all expanded in the API page.
• [TODO:] makes sure the docbuild occurs and docs are pushed to gh-pages!

[codecov-commenter]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (871f5ef) 85.23% compared to head (f49fa0e) 85.23%. Report is 3 commits behind head on main.

:exclamation: Current head f49fa0e differs from pull request most recent head 431b9f8. Consider uploading reports for the commit 431b9f8 to get more accurate results

Files	Patch %	Lines
src/core/agents.jl	87.50%	8 Missing :warning:
src/core/model_standard.jl	80.00%	4 Missing :warning:
src/simulations/sample.jl	86.66%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #969 +/- ## ========================================== - Coverage 85.23% 85.23% -0.01% ========================================== Files 36 36 Lines 2533 2587 +54 ========================================== + Hits 2159 2205 +46 - Misses 374 382 +8 ```

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[Tortar]

Great improvements. I will take the time to review it soon.

Would you believe that this PR took me more than 8 hours?!?!?!

I quite believe it. I would have estimated the same time if I would have been done myself, but with a worse result probably :D

[Tortar]

I think that this proves that #963 can be closed. Also, I think it is a good time to make sure the docbuild happens so that we can be sure that everything works properly, hope you find it easy enough to fix it

[Datseris]

I will do my best! I anyways spend time the last weeks fixing docs in many other JuliaDynamics packages so I am sure I can get it to work

[Tortar]

Great! the new theme is cool but it seems the new zoom is a bit off:

(I checked this happens also with your new DynamicalSystems docs), does it happen to you too? Or the zoom is right on your browser? I'm using Firefox at the moment, maybe it is an issue in documenter though

---

Anyway I think you need to merge main into the branch to fix the test failing, but I'm kinda of sure this will not happen where you merge so you could also not fix it

[Tortar]

mmh I see you load some css, so I think the zoom problem should be there maybe

[Datseris]

yeah with documenter v1 some things got broken, but i don't know how to fix them yet. I have asked online.

[Datseris]

(we will of course try to fix everything before a final release, but it may be that some things will remain "somewhat broken")