Programmable Button API refactor and improve docs

[fauxpark]

Description

This feature is very spammy as the reports are sent in keyboard_task() regardless of whether there are any changes. I've removed this and added two new functions, programmable_button_un/register() which call _add/remove() and then _flush() to send the report. This way you can still leverage the API to programmatically modify multiple button states in a single report, but the keycodes will send a report immediately.

The documentation has also been cleaned up, and particularly I have added a note regarding support for this feature on Windows and macOS.

Types of Changes

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

Issues Fixed or Closed by This PR

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).