Update private chat bot use case to use ollama by default

[matevz]

Update https://docs.oasis.io/build/use-cases/tgbot to use Ollama by default. It's just a matter of setting OLLAMA_MODEL and (optionally) OLLAMA_SYSTEM_PROMPT. Then, merge the ollama branch in https://github.com/oasisprotocol/demo-rofl-tgbot.