Document the crates.io publish process and prepare crates for publishing

[lonerthefirst3-sudo]

Summary

Prepare the workspace crates for eventual publication to crates.io and document the release process.

What to do

1. Check Cargo.toml metadata in each publishable crate (soroban-guard-checks, soroban-guard-analyzer, soroban-guard-cli):

   • description field
   • license field
   • repository field
   • keywords and categories
   • readme pointing to README.md
   • publish = true (or remove the publish = false restriction)

2. Add a CHANGELOG.md with an initial Unreleased section.

3. Document the release steps in CONTRIBUTING.md or a new docs/release.md:

   • Bump version in workspace Cargo.toml
   • Run cargo publish -p soroban-guard-checks, then -p soroban-guard-analyzer, then -p soroban-guard-cli
   • Tag the release in git

Why it matters

Contributors cannot depend on the crates via crates.io today. Publishing enables cargo install soroban-guard-cli and downstream library use.