Upgrade guide from v2 to v3

This is an upgrade guide to upgrade your Homie devices from v2 to v3.

New convention

The Homie convention has been revised to v3 to be more extensible and introspectable. Be sure to check it out.

API changes in the sketch

1. Constructor of HomieNode needs third mandatory param const char* type: E.g. HomieNode lightNode("light", "Light"); -> HomieNode lightNode("light", "Light", "switch");.
2. Signature of handleInput has changed to: handleInput(const HomieRange& range, const String& property, const String& value) See the Ping example for the current callback shape.
3. Review every advertised property id against the Homie 3.0.1 id rules. This fork keeps legacy ids for compatibility but logs a warning when an id is not convention-compliant.
4. If you rely on overwriteSetter(true), use setSetRetained() when the mirrored /set topic needs a different retained flag from the main property publish.