docs: Create a user-space API guide

This is meant to be the place for documentation relevant to application
developers. It's empty for the moment, but at least we have a place now!

Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/index.rst

···
 
    admin-guide/index
 
+Application-developer documentation
+-----------------------------------
+
+The user-space API manual gathers together documents describing aspects of
+the kernel interface as seen by application developers.
+
+.. toctree::
+   :maxdepth: 2
+
+   userspace-api/index	      
+
+
 Introduction to kernel development
 ----------------------------------
 

Documentation/userspace-api/conf.py

···
+# -*- coding: utf-8; mode: python -*-
+
+project = "The Linux kernel user-space API guide"
+
+tags.add("subproject")
+
+latex_documents = [
+    ('index', 'userspace-api.tex', project,
+     'The kernel development community', 'manual'),
+]

Documentation/userspace-api/index.rst

···
+=====================================
+The Linux kernel user-space API guide
+=====================================
+
+.. _man-pages: https://www.kernel.org/doc/man-pages/
+
+While much of the kernel's user-space API is documented elsewhere
+(particularly in the man-pages_ project), some user-space information can
+also be found in the kernel tree itself.  This manual is intended to be the
+place where this information is gathered.
+
+.. class:: toc-title
+
+	   Table of contents
+
+.. toctree::
+   :maxdepth: 2
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`