PLT-7685 Documentation for open roles.

[bwbush]

Documentation and an example of using open roles in Marlowe Runtime.

Pre-submit checklist:

• Branch
  • Tests are provided (if possible)
  • Test report is updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
  • Relevant tickets are mentioned in commit messages
  • Formatting, PNG optimization, etc. are updated
  • Operables are updated with changes to executable command line options.
  • Deploy charts updated with changes to operables.
• PR
  • Self-reviewed the diff
  • Useful pull request description
    • Review required
    • Substantial changes to code, test, or documentation
    • Change made to Marlowe validator (@bwbush and @palas must be included as reviewers)
    • Review not required
    • Minor changes to non-critical code, documentation, nix derivations, configuration files, or scripts
    • Formatting, spelling, grammar, or reorganization
  • Reviewer requested