docs: Organize core-api/index.rst

The core-api manual has become a big, disorganized mess.  Try to bring a
small amount of order to it by organizing the documents into
subcategories.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/index.rst

@@ -8,42 +8,81 @@ This is the beginning of a manual for core kernel APIs.  The conversion
 Core utilities
 ==============
 
+This section has general and "core core" documentation.  The first is a
+massive grab-bag of kerneldoc info left over from the docbook days; it
+should really be broken up someday when somebody finds the energy to do
+it.
+
 .. toctree::
    :maxdepth: 1
 
    kernel-api
+   workqueue
+   printk-formats
+   symbol-namespaces
+
+Data structures and low-level utilities
+=======================================
+
+Library functionality that is used throughout the kernel.
+
+.. toctree::
+   :maxdepth: 1
+
    kobject
    assoc_array
+   xarray
+   idr
+   circular-buffers
+   generic-radix-tree
+   packing
+   timekeeping
+   errseq
+
+Concurrency primitives
+======================
+
+How Linux keeps everything from happening at the same time.  See
+:doc:`/locking/index` for more related documentation.
+
+.. toctree::
+   :maxdepth: 1
+
    atomic_ops
-   cachetlb
    refcount-vs-atomic
-   cpu_hotplug
-   idr
    local_ops
-   workqueue
+   padata
+   ../RCU/index
+
+Low-level hardware management
+=============================
+
+Cache management, managing CPU hotplug, etc.
+
+.. toctree::
+   :maxdepth: 1
+
+   cachetlb
+   cpu_hotplug
+   memory-hotplug
    genericirq
-   xarray
-   librs
-   genalloc
-   errseq
-   packing
-   printk-formats
-   circular-buffers
-   generic-radix-tree
+   protection-keys
+
+Memory management
+=================
+
+How to allocate and use memory in the kernel.  Note that there is a lot
+more memory-management documentation in :doc:`/vm/index`.
+
+.. toctree::
+   :maxdepth: 1
+
    memory-allocation
    mm-api
+   genalloc
    pin_user_pages
-   gfp_mask-from-fs-io
-   timekeeping
    boot-time-mm
-   memory-hotplug
-   protection-keys
-   ../RCU/index
-   gcc-plugins
-   symbol-namespaces
-   padata
-   ioctl
-
+   gfp_mask-from-fs-io
 
 Interfaces for kernel debugging
 ===============================
@@ -54,6 +93,18 @@ Interfaces for kernel debugging
    debug-objects
    tracepoint
 
+Everything else
+===============
+
+Documents that don't fit elsewhere or which have yet to be categorized.
+
+.. toctree::
+   :maxdepth: 1
+
+   librs
+   gcc-plugins
+   ioctl
+
 .. only:: subproject and html
 
    Indices