docs(e2e): write contributor guide for e2e_test crate (backlog#1153 infra-10)
Turn the e2e_test README skeleton into a dense, link-heavy contributor guide:
crate overview + grouped module map, how to run (whole crate / one module /
e2e-smoke profile / ILM serial lane / protocols suite), #[ignore] semantics as
a pointer to live sources, how to add a test (RustFSTestEnvironment +
RustFSTestClusterEnvironment, common.rs/chaos.rs helper inventory, isolation
rules, #[serial]-vs-nextest reality), a CI subset map table, and a
troubleshooting section (stale binary / RUSTFS_TEST_PORT / orphan processes).
Keeps the ci-4 "CI smoke subset" section intact and restructures around it.
Adds a reciprocal link from crates/e2e_test/AGENTS.md.
Refs rustfs/backlog#1153 (infra-10), #1155.
Zhengchao An