Each maintainer can have an optional URL. There are seceral use
cases for providing a URL instead of an email address:
- The case where a maintainer wants to hide their email address
  behind a webform for personal security
- When the maintainer is an organization and feedback should
  go to an issue queue

This change:
- Adds the proto field for url
- Updates the generated files for the chart proto
- Includes linting for the url
- Updates the docs to include the url

Closes #3056