doc: blobs: fixes and tweaks

From a user perspective, it doesn't really matter that west blobs is
an extension. And again from a user perspective, the fact that the
command is called "blobs" is much more important than knowing that
it's an extension. Similarly, the fact that we host it in zephyr and
why isn't super relevant.

Re-work the docs so that we refer to the command as "west blobs"
instead of "a/the west extension" or "west", for ease of reading.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>