docs(contracts): rustdoc comments for all public functions and modules (#9)

[ZacLou]

Closes #9

Adds rustdoc comments and module-level documentation across all 5 cooperative contracts:

• treasury: module doc + complete /// docs for every pub fn.
• loan: module doc + docs for initialize, request_loan, approve_loan, repay, get_loans, get_loan.
• voting: module doc + docs for initialize, create_proposal, vote, finalize, get_proposals, get_votes.
• governance: module doc + docs for initialize, update_rules, get_rules.
• dividend: module doc + docs for initialize, distribute, get_distributions.

Also adds a docs job to .github/workflows/test.yml that runs cargo doc --no-deps --workspace.

Note: local cargo doc on this Windows environment hits a pre-existing ed25519-dalek / soroban-env-host trait-bound compile error unrelated to documentation. The documentation changes themselves are syntax-only and the new CI job should validate cargo doc --no-deps in the project's supported environment.