Add "license" field to Community Module JSON schema.

[getreuer]

This PR adds an optional "license" field to indicate a license in the qmk_module.json file of a community module.

Description

It is desirable that module authors indicate a license to clarify how the module may be used and shared, to foster collaboration and wider adoption.

• A simple string "license" field is added to the schema for qmk_module.json.

• In the documentation, I recommend that this field is a SPDX license identifier like "Apache-2.0" or "GPL-2.0-or-later" if possible. Even so, the field allows an open-ended text description in case of more complex licensing situations.

• I updated the qmk/hello_world and qmk/super_alt_tab modules with "license": "GPL-2.0-or-later", matching the license on their source files.

• While I was at it, I updated the docs to mention the (currently undocumented) "url" field, which is something else I have been wanting.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).