Add a guide for Mapbox functionality

[janbaykara]

There's already one for Data Pipelining.

• Simple: explain the primitives + how how they fit together
• Complex: how to extend them (e.g. Stimulus controllers, alternative templatetag implementations)
• Real code examples that pique your curiosity, showing how Stimulus controllers + Django views and templates come together to make something visually interesting:
  • https://github.com/commonknowledge/leftbookclub/blob/main/app/templates/app/includes/event_list_and_map.html