paper: Add code examples and clarify terminology

[EwoutH]

This PR implements some last feedback from @jofmi and @martibosch on our JOSS paper for Mesa.

Each commit processes one feedback point:

1. a30c41d: Add minimal code examples for Model and Agent init
   • Added simple Python code examples showing basic structure of Model and Agent classes
2. ae216cd: Add that we refer to Agent subclasses as types of agents
   • Clarified terminology by explaining that agent subclasses are often referred to as "types" of agents
3. 6baa2dc: Explain VoronoiMesh
   • Added brief explanation of what Voronoi tessellations are
4. b451d9f: Integrate discrete and continuous time feedback
   • Enhanced explanation of Mesa's time advancement capabilities with clearer distinction between discrete and continuous time
5. cacd3a0: Update SolaraViz example code to align with screenshot
   • Made the visualization example code more descriptive with improved parameter names and comments
6. 2b10107: paper: Add link to extensions documentation
   • Adds a hyperlink to our new extensions page on Readthedocs

Rendered paper (PDF).

Part of #2666.

[tpike3]

LGTM--- Thanks for all your hard work on this @EwoutH

[EwoutH]

Thanks for reviewing! @quaquel are you good with the addition of the discrete and continuous terminology?

[EwoutH]

@quaquel thanks, it was a reviewer suggestion but I didn’t think it aligned with ABM terminology, like you confirmed.

Thanks for the specific suggestions!