docs: Mention demo-starter-go and demo-starter-py

[matevz]

Fixes https://github.com/oasisprotocol/demo-starter-go/issues/2.

This PR:

• adds demo-starter-go and demo-starter-py references
• makes usage of :::info Example: consistent

Merge after https://github.com/oasisprotocol/demo-starter-py/pull/1.

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[matevz]

Thanks. Merged.