docs: define the protocol upgrade policy

[MavenRain]

Issue #448 calls for a pre-mainnet policy for changes to consensus rules on an existing network. Document that those changes must use a future epoch-versioned fork mechanism that preserves chain identity and verifiable history, and link the policy from the documentation index and operational update guidance.

The document explains why bumping PROTOCOL_VERSION alone breaks peer authentication and historical checkpoint verification, then records the requirements for coordinated activation and historical signature domains. It explicitly marks the mechanism as unimplemented and leaves its design under #448.

Refs #448.

Validation:

• git diff --check
• cargo +nightly fmt --all --check
• Checked all 14 local Markdown links in the three changed documents.

Documentation only; no Rust compilation or runtime tests. The domain and checkpoint claims were checked against the source.