[Genteure]

Allow the url property to be omitted from board, shield, and interconnect metadata.

For a lot of one-off custom keyboards, there's not really a page for them. For these configs people usually use the zmk-config themselves as the url, or leave url empty/missing.

zmk-cli already uses str | None for url.

ZMK docs hardware list also changed in this PR.

I'm not aware of any other ZMK metadata consumer.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.