Docs: describe registering as a proof producer about proof-market-toolchain

[alexandra-mara]

Just a draft

Register as a proof producer

If you wish to submit generated proofs to the Proof Market,
you need to also register as a proof producer:

foo@bar:~$ python3 signup.py producer --url https://example.com

This will return producer metadata in JSON format:

{
  "_key": "...",
  "_id": "producer/...",
  "_rev": "_fneT7J2---",
  "description": "Generic Producer",
  "url": "https://example.com",
  "name": "username",
  "createdOn": 1677588821180,
  "updatedOn": 1677588821180
}

[alexandra-mara]

@NickVolynkin for User Guides -> Proof Producer fixed by #71 - Check out the current version at https://docs.nil.foundation/proof-market/market/user-guides/sign-up#proof-producer

If it looks okay, we can just add the same part to CLI guides