docs: add stubs for all commands

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/buildx_prune.md

@@ -0,0 +1,19 @@
+# buildx prune
+
+```
+Usage:  docker buildx prune
+
+Remove build cache
+
+Options:
+  -a, --all                  Remove all unused images, not just dangling ones
+      --builder string       Override the configured builder instance
+      --filter filter        Provide filter values (e.g. 'until=24h')
+  -f, --force                Do not prompt for confirmation
+      --keep-storage bytes   Amount of disk space to keep for cache
+      --verbose              Provide a more verbose output
+```
+
+## Description
+
+Remove build cache