docs: More docs and improvements of existing docs

- Tried not to leave a single public function undocumented, though did not double-document public function with obvious names
- Remove all mentions of the strategy selector in favour of the strategy registry (to avoid confusion)

src/encoding/evm/swap_encoder/builder.rs

@@ -4,6 +4,7 @@ use crate::encoding::{
     swap_encoder::SwapEncoder,
 };
 
+/// Builds a `SwapEncoder` for the given protocol system and executor address.
 pub struct SwapEncoderBuilder {
     protocol_system: String,
     executor_address: String,