Add more useful queries in documentation

[khwerhahn]

Problem Report
To understand db-sync better and get devs working faster it would be great to increase the amount of 'useful queries'. Maybe taking TapTools as a good base. It would really help to understand the relationships better.

1. Current holdings of any address and stakekey
2. Past holdings of any address and stakekey
3. Get asset details (names, logos, etc)

Maybe also some advanced stuff. I don't know if they are possible just from db-sync?

1. Calculate prices for assets (maybe start with Minswap as an example?)
2. A query selecting only "NFTs"
3. Unique wallets trading
4. Trading volume

I'm sure there are many more useful ones.