[docs] Adds a description of all tables & general workflow

[frgfm]

Similar to UML of tables, there is no clear description of table logics for non-expert users.

We should document this

What we need:

• [x] Table description & logic (#227)
• [x] Event lifecycle (#227)
• [x] Table UML diagram (#228)
• [x] #168
• [x] External services (storage, DB) (#227)
• [x] #221
• [x] #225
• [ ] #230

[fe51]

Thank for opening this,

What description do you think is needed here ?

• [x] An entity diagram of tables
• [ ] A file describing tables and its fields ? Do you have an idea of the relevant format ?