Fix sphinx-build warnings and turn warnings into errors

[benhoyt]

Fix the 100+ warnings when generating docs (and make warnings errors)

• Hack-fix issue with hyphenated names in TypedDict fields
• Ignore several private classes in Sphinx config's nitpick_ignore list
• Make several things public as they're used in public methods:
  • Serializable protocol
  • Pebble plan/layer TypesDicts (and sub-dicts), as people do build those in Python
• Remove TypedDicts for charm metadata (it's not useful, as users don't build their charm metadata in Python; it's in YAML).
• Remove TypedDicts for event snapshots, as IMO these don't add enough value to pay for themselves.
• Replace some named types with spelled-out types for clarity, eg: _SerializedData is just Dict[str, Any]; Numerical is just Union[int, float].
• Use simpler Dict[str, Any] rather than _JsonObject (the more complex, "exact" type doesn't pay for itself).
• Change EventSource and BoundEvent to non-generic classes to simplify. (TODO: not sure if this is a good idea)

Part of: #920