docs: feegrant with curl

[jcstein]

add to feegrant guide, or make new guide on doing with curl

1. start local light node for mocha:
   celestia light start --p2p.network mocha --core.ip rpc-mocha.pops.one
2. in 2nd terminal, export auth token, so you can send curls without disabling auth.
   CELESTIA_NODE_AUTH_TOKEN=$(celestia light auth admin --p2p.network mocha)
   (in theory, you could disable auth when starting node with --rpc.skip-auth)
3. get my key's address of running node in same terminal. set as variable
   GRANTER_ADDRESS=$(celestia state account-address | jq -r .result) && echo $GRANTER_ADDRESS
   also set grantee address (the address you want to have the grant)
   GRANTEE_ADDRESS=celestia1377k5an3f94v6wyaceu0cf4nq6gk2jtpc46g7h
4. send curl with those vars

curl -H "Content-Type: application/json" -H "Authorization: Bearer $CELESTIA_NODE_AUTH_TOKEN" -X POST --data "{
  \"id\": 1,
  \"jsonrpc\": \"2.0\",
  \"method\": \"state.GrantFee\",
  \"params\": [
    \"$GRANTEE_ADDRESS\",
    \"42\",
    {
      \"gas_price\": 0.002,
      \"is_gas_price_set\": true,
      \"gas\": 142225,
      \"key_name\": \"my_celes_key\",
      \"signer_address\": \"$GRANTER_ADDRESS\",
      \"fee_granter_address\": \"$GRANTER_ADDRESS\"
    }
  ]
}" http://localhost:26658/

5. see result on celenium

{"id":1,"jsonrpc":"2.0","result":{"height":3068554,"txhash":"4AAFDA4CE622C56CE9681B7E3BE7773E3B36ABE6C02BD66111BD2F2491011FD2","logs":null,"events":null}}