Improve docs for `ENSINDEXER_SCHEMA_NAME` environment variable

[lightwalker-eth]

The way this is currently documented in our README.md just caught my attention. We can and should do more and better here.

Very open to adding links to Ponder documentation. But it shouldn't be a mystery where the "linked documentation" is.

Please review apps/ensnode/.env.local.example for some inspirations.

Related issue: #22

[lightwalker-eth]

Lots of additional context shared in Slack here: https://namehash.slack.com/archives/C086Z6FNBHN/p1764678314755089?thread_ts=1764618040.430819&cid=C086Z6FNBHN