Add mainnet docs + refacto node running docs

[Leo-Besancon]

Related to: https://github.com/massalabs/docs/issues/254

DONE:

• [x] Add Mainnet config and tabs when relevant
• [x] Add Mainnet description in Public-Networks
• [x] Put ChainId doc in sandbox node page
• [x] Refactored a bit the node running guide (for normal node runners)

TODO for initial node runners:

• [ ] make sure everything is working if we follow the steps described in docs/node/initial.mdx
• [ ] proof-read everything

TODO for normal node runners:

• [x] fill the TODOs on how to import saved wallets
• [x] add node routability setup at node install phase. Insist that without routability, their node will be unstable through lack of connectivity
  • [x] for routability, DO NOT FORGET TO SET THE IP ADDRESS IN THE CONFIG
  • [x] tell how to check that it is defined (eg. I think the client tells you your IP address on top of get_info (?))
• [x] check that all links work
• [x] try to render and see how it looks
• [x] add a last phase where you check the status of your node:
  • [x] routability check using an external service like https://portchecker.co/
  • [x] make sure your node is connected to peers
• [ ] make sure everything is working if we follow the steps described in docs/node/home.mdx
• [ ] proof-read everything