docs: add Example App section and link to main README [skip ci]

docs: rewrite example README with correct setup flow [skip ci]

chore: replace ngrok URL with placeholder in manifest [skip ci]

fix: correct modal view title/submit type fields and add error logging

feat: add debug logging for handler registration

fix: add catch-all MessageEvent handler to suppress Bolt warning

fix: add request URLs to manifest for events, interactivity, and commands [skip ci]

fix: add _metadata to manifest.json [skip ci]

fix: remove empty usage_hint from manifest [skip ci]

feat: add Slack app manifest for example app [skip ci]

feat: add example app demonstrating all starter features

chore: remove explicit empty constructors

docs: add javadoc to all public API classes and methods

docs: update CHANGELOG and README for 0.3.0 release [skip ci]

docs: add @Block and @ActionId documentation to README [skip ci]