doc: move pstore.rst to usage/pstore.rst

Let's have a separate chapter dedicated to using U-Boot.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/index.rst b/doc/index.rst
index 68a083b..4c44955 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -25,6 +25,7 @@
    :maxdepth: 2
 
    build/index
+   usage/index
 
 Developer-oriented documentation
 --------------------------------
@@ -109,13 +110,6 @@
 
    android/index
 
-Command line
-------------
-.. toctree::
-   :maxdepth: 2
-
-   pstore.rst
-
 Indices and tables
 ==================