Add xDAI wallet/node info to Perpetual Protocol connector docs

[fengtality]

Why

To use the new Perpetual Protocol connector, users need to connect their xDAI wallet and node. Currently they can't change the node yet, but in the next release they will be able to. However, there's little info on https://docs.hummingbot.io/protocol-connectors/perp-fi/ that tells them how to do this.

What

1. Speak to @vic-en to understand what changes he plans to make to Perpetual Protocol connector before April release, especially around node/wallet connection.
2. Add information on how to connect xDAI wallet
3. Add information on how to change xDAI node (join Perpetual Protocol Discord's #hummingbot channel for more info)
4. Add information on how users can switch between testnet and mainnet