doc: Cleanup env variables/ cli arguments info, add prometheus info

[x] update the readme with the comment that the user can use both cli or env args
[x] update the readme with the info about prometheus (think about what should be mentioned in the doc)
[x] update the readme with the list of all the args with the short explanation if needed (if it's not intuitive)
[x] update the readme with the examples of the args provided, both env and cli. Please do not share here any secrets

We need to do the same for balances as well

near / near-indexer-events