doc: add project documents

[sergey-melnychuk]

• code coverage
• architecture

[ICavlek]

LGTM, especially great work on the architecture document :) One small change that might be updated in the architecture.md, not all functions except starknet_call are proxied. starknet_getStorageAt also calls pathfinder_getProof and verifies it.

[sergey-melnychuk]

Coverage Results:
|| Tested/Total Lines:
|| src/client.rs: 9/63
|| src/config.rs: 21/59
|| src/eth.rs: 0/78
|| src/exe/err.rs: 0/6
|| src/exe/map.rs: 48/50
|| src/exe/mod.rs: 75/127
|| src/proof.rs: 86/157
|| src/rpc.rs: 114/154
|| src/util.rs: 13/14
||
51.69% coverage, 366/708 lines covered