feat(extensions): most of the extensions implementation, by kdb424

docs/extensions.md

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+# Extensions
+Extensions add features that change the experience, but not the core features of
+the keyboard. They are meant to be easy to add, and create your own. These live in
+a sandbox to help prevent any bad code from crashing your keyboard.
+
+## Core Extensions
+These extensions are proveded in all builds and can be enabled. Currently offered
+extensions are
+
+- [International](international.md): Adds international keycodes
+- [LED](led.md): Adds backlight support. This is for monocolor backlight, not RGB
+- [MediaKeys](media_keys.md): Adds support for media keys such as volume
+- [RGB](rgb.md): RGB lighting for underglow. Will work on most matrix RGB as will
+be treated the same as underglow.