Use ownable in examples

[jsmaxi]

Fixes #225

Added ownable where it made sense.

Some examples use the access_control module instead of ownable, which I think is fine.

---

Notes

• Combining ownable with pausable is not very elegant in practice. For example:

fn pause(e: &Env, caller: Address) {
    let owner: Address = enforce_owner_auth(e);
    if owner != caller {
        panic_with_error!(e, ExampleContractError::Unauthorized);
    }

    pausable::pause(e);
}

• In the vault example, ownable was added, but it isn’t actually used to authorize functions, just included for demonstration purposes.

• I didn’t touch the upgradeable contract examples, since you may want to keep their current wasm unchanged.

• I’ve also added examples/DEPLOY.md, which contains sample Stellar CLI deploy commands for the example contracts.

PR Checklist

• Tests - tests are passing, though no new tests have been added.
• Documentation - documentation appears to have been moved to another repository.

Looking forward to your review and suggestions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.72%. Comparing base (372a162) to head (5d3e911).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #487   +/-   ##
=======================================
  Coverage   94.72%   94.72%           
=======================================
  Files          74       74           
  Lines        5020     5020           
=======================================
  Hits         4755     4755           
  Misses        265      265           

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