btrfs-progs: docs: merge project documentation section to overview

The links to repositories and contributors is hard to find as it's below
the feature list. Move it up as it is for overview.

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

Documentation/index.rst

@@ -15,8 +15,10 @@ Welcome to BTRFS documentation!
    CHANGES
    Feature-by-version
    Kernel-by-version
+   Contributors
    Glossary
    INSTALL
+   Source-repositories
 
 .. toctree::
    :maxdepth: 1
@@ -46,13 +48,6 @@ Welcome to BTRFS documentation!
    Volume-management
    Zoned-mode
 
-.. toctree::
-   :maxdepth: 1
-   :caption: Project information
-
-   Source-repositories
-   Contributors
-
 .. toctree::
    :maxdepth: 1
    :caption: Developer documentation