docs: document development version convention

[rsheyd]

What changed

• document the -dev.N SemVer prerelease convention for identifiable development builds;
• explain when to increment the development-build number;
• clarify that Cargo.toml and Cargo.lock should be updated together;
• describe replacing the suffix with the final release version during release preparation.

Why

Contributors may need to install or share distinguishable development builds without adding build-time Git metadata or changing the version for every feature commit. A documented -dev.N convention provides a simple, standard way to identify those builds.

Validation

• git diff --check
• documentation-only change; Rust tests were not run