Add relevant code samples to the documentation (Flutter)

[AndyNicks]

As we have added support for more parts of the 1.2 spec we need to update SDK documentation and make sure that relevant code samples are added to the docs.

[AndyNicks]

1. First, go through the docs and see where code snippets for flutter are missing based on the 1.2 release.
2. Fill in the gaps

[tiholic]

for tracking: https://github.com/ably/docs/issues/1153