Add section "Common Misconceptions"

[cmichi]

We should add a section "Common Misconceptions" or "Common (Beginner?) Mistakes" with the things we see in real-world contracts. Such as:

• A misconception that we often see is that developers think the smart contract is the actual interface for the user.
  That's why we often see mnemonic String types returned (in the worst case verbatim error messages instead of error codes).

• We should explain why contract size is important.
  We sometimes get questions like "My 100K contract can't be deployed". This is usually due to some third party crate dependency with default features enabled. Oftentimes the size can be reduced by tuning the features of that dep or using the built-in ink! API instead (oftentimes it's crypto/hashing dependencies).

• We should explain better why String should be avoided and especially what the alternatives are.
  We already have a section How do I use String in my contract? in our FAQ, but it goes in a bit of a different direction. Using String pulls in logic needed for e.g. UTF-8 handling, which one might not need, but it blows up the contract file size. Also, for String dynamic allocation is needed, increasing the file size of the contract as well and being expensive in terms of performance. A use-case I have now seen a couple times is to use String to generate some uuid, we should look into an alternative for doing that.

[SkymanOne]

Can be documented as "Best practices in ink!"

We should have this soon.