docs: standardize format for usage

Use the usage output of `--help` for each subcommand, to make
sure all flags/options are included on the page, and to make
it easier to keep docs in sync.

Note that the usage output is only used when reading these
docs on GitHub; docs.docker.com only consumes the "description"
and "example" sections (when present), and generates flag information
and usage output from source

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

docs/reference/buildx_imagetools_create.md

@@ -1,13 +1,17 @@
-# `buildx imagetools create [OPTIONS] [SOURCE] [SOURCE...]`
+# buildx imagetools create
+
+```
+Usage:  docker buildx imagetools create [OPTIONS] [SOURCE] [SOURCE...]
+
+Create a new image based on source images
 
 Options:
-
-| Flag | Description |
-| --- | --- |
-|      --append            | Append to existing manifest
-|      --dry-run           | Show final image instead of pushing
-|  -f, --file stringArray  | Read source descriptor from file
-|  -t, --tag stringArray   | Set reference for new image
+      --append             Append to existing manifest
+      --builder string     Override the configured builder instance
+      --dry-run            Show final image instead of pushing
+  -f, --file stringArray   Read source descriptor from file
+  -t, --tag stringArray    Set reference for new image
+```
 
 ## Description