Document what data is stored in the DB

[MegaRedHand]

The data we store in the DB is getting complex. We should add a markdown document explaining which data is stored and how.

[Aliemeka]

Can I work on this. Please I like to see the recommended format the team uses and also you can add further important details on key aspects/implementations that needs to be mentioned in the doc.

[MegaRedHand]

Sure, go ahead! We don't have a recommended format yet, but ethrex has a part about contributing to the documentation, with information on how to work with mdbook.

As for key aspects/implementations, we mainly want to touch upon the Store/StorageBackend split, and what we store in each of the tables. That's briefly documented in the Table enum, but it should be fleshed out some more.

If you have any question or need any pointers, let me know!