Docs `Precompiles` and `System calls`

[jorbush]

Precompiles

• [x] what are precompiles?
• [x] how calls to precompiles work
• [x] use keccak as an example as it's used on every deployment (it goes to the keccak.yul contract which then uses the precompile opcode)

  System Calls

• [x] What are system contracts?
• [x] What's a system call?
• [x] Show some examples (deployer, nonce holder, L2BaseToken) and what they're used for.