fix(docs): update sponsored fpc address docs

[critesjosh]

Summary

• Update the testnet SponsoredFPC address in the networks table and getting-started guides.
• Adjust release docs guidance to reflect that SponsoredFPC is deployed on testnet and devnet, but not mainnet.

Validation

• yarn spellcheck from docs/

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 1 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/d3c05119fba6aa8c�d3c05119fba6aa8c8;;�): yarn-project/kv-store/scripts/run_test.sh src/bench/sqlite-opfs-encrypted/map_bench.test.ts (1s) (code: 0)