[docs] Improvements to Racer model docs

[ericyhwang]

• Overall:
  • Document *Promised methods added in [email protected], with examples
  • Change the very outdated "store" term to use "backend" instead
• Main Models page:
  • Add textual overview of important sub-pages
  • Rewrite docs on creating models for clarity
  • Add docs for model.close()
• Contexts:
  • Small rewrites for clarity
• Events:
  • Recommend that new code use {useEventObjects: true}
  • Mention that TS types only support that event-objects callback format
  • Hide full docs on legacy event callbacks by default
• Getters:
  • Add docs for getOrThrow and getOrDefault added in [email protected]
  • Make a louder warning about in-place modififications of values returned by non-copying get* methods
• Mutators:
  • Add section about error handling
  • Rewrite info about confirming mutations, now in its own section with examples
  • Add docs about *Promised mutator methods in that section
  • Document whenNothingPending() with appropriate warnings
• Paths:
  • Include full example of model data structure
  • Update docs on local vs remote collections for clarity
• Queries:
  • Add explanations on how to subscribe/fetch a query, via both callback and promise APIs
  • Add examples
• Reactive functions:
  • Move model.evaluate to its own paragraph, to make it more clear what options applies to it vs model.start
  • Minor wording updates and clarifications
• References:
  • Mention similarity to filesystem symlinks