doc: Move blobstore docs from modules to Programmer Guides

It's at least the beginning of a Programmer Guide.

Change-Id: Ifb0a6a1eefde83ef398f0dc6472c57e9faf4eacc
Signed-off-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-on: https://review.gerrithub.io/398104
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>

doc/index.md

@@ -32,13 +32,12 @@
 - @ref directory_structure
 - [Public API header files](files.html)
 - @ref event
+- @ref blob
 
 # Modules {#modules}
 
 - @ref nvme
 - @ref ioat
-- @ref blob
-
 - @ref virtio
 
 # Tools {#tools}