docs: audit and update documentation for release readiness (closes #114)

[joshrotenberg]

Summary

Documentation audit and update for release readiness. Covers all significant
changes since v0.9.5 that weren't yet reflected in docs.

Changes:

• CHANGELOG.md: add missing [Unreleased] entries for simplified alias façade (api(consistency): finish harmonized handler surface across remaining domains #65 / PR feat(api): add simplified alias methods across remaining domain handlers (closes #65) #112) and Python parity expansion (python(parity): define and close the gap between Rust client and Python bindings #66 / PR feat(python): expand bindings to cover all major read domains, document parity scope (closes #66) #113); consolidate duplicate ### Changed sections; reorder per Keep a Changelog convention (Added → Changed → Fixed)
• README.md: update Quick Start and examples to prefer simplified alias methods; update API coverage section to reflect 100% spec coverage; clarify empty allowlists
• src/lib.rs: update handler overview table to show individual connectivity handlers (VpcPeeringHandler, TransitGatewayHandler, PscHandler, PrivateLinkHandler) and add CostReportHandler; update subscription examples to use list() alias; update feature list
• python/README.md: fix Out-of-scope section — put/patch are not exposed as raw HTTP helpers in the Python bindings
• examples/basic.rs: prefer list() over get_all_subscriptions()
• examples/databases.rs: prefer list() over get_subscription_databases() with updated iteration

Test plan

• cargo doc --no-deps --all-features 2>&1 | grep warning produces no output
• cargo test --workspace passes
• cargo clippy --workspace --all-targets -- -D warnings is clean
• CHANGELOG [Unreleased] has entries for api(consistency): finish harmonized handler surface across remaining domains #65 and python(parity): define and close the gap between Rust client and Python bindings #66
• README examples use the simplified alias API
• python/README.md coverage table matches python/src/client.rs