docs: add v0.3 to v0.4 migration guide (#51)

## Summary
- add a new `v0.3` to `v0.4` migration guide under `docs/`
- document the main source-level migration areas from `v0.3.0` to
current `main`, especially Agent V1 settings, renamed history/context
helper types, manage key request renames, and audio output container
typing
- update the README migration section so the new guide is linked as the
current migration path

## Notes
- guide content was written from the actual `v0.3.0..main` diff, not
just commit titles
- docs-only change

Author: GregHolmes

README.md

@@ -16,7 +16,8 @@ You can learn more about the Deepgram API at [developers.deepgram.com](https://d
 
 ### Migrating Between Versions
 
-- [v0.2 to v0.3](./docs/Migrating-v0.2-to-v0.3.md) (current)
+- [v0.3 to v0.4](./docs/Migrating-v0.3-to-v0.4.md) (current)
+- [v0.2 to v0.3](./docs/Migrating-v0.2-to-v0.3.md)
 
 ## Installation