This was broken when updating the documentation targets for the Sphinx
build, and moving from %docs target pattern to explicitly listed
targets.

Cc: Markus Heiser <markus.heiser@darmarit.de>
Cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Fixes: 22cba31b ("Documentation/sphinx: add basic working Sphinx configuration and build")
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

While there's slight overlap with the DocBook help now, this can stay
intact when the DocBook help goes away.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

Read the version and release from the top level Makefile (for use when
Sphinx is invoked directly, by e.g. Read the Docs), but override them
via Sphinx command line arguments in a normal documentation build.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

Tell Sphinx where to find the extension, and pass on the kernel src tree
and kernel-doc paths to the extension.

With this, any .rst files under Documentation may contain the kernel-doc
rst directive to include kernel-doc documentation from any source file.

While building, it may be handy to pass kernel-doc extension
configuration on the command line. For example, 'make SPHINXOPTS="-D
kerneldoc_verbosity=0" htmldocs' silences all stderr output from
kernel-doc when the kernel-doc exit code is 0. (The stderr will be
logged unconditionally when the exit code is non-zero.)
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

Add basic configuration and makefile to build documentation from any
.rst files under Documentation using Sphinx. For starters, there's just
the placeholder index.rst.

At the top level Makefile, hook Sphinx documentation targets alongside
(but independent of) the DocBook toolchain, having both be run on the
various 'make *docs' targets.

All Sphinx processing is placed into Documentation/Makefile.sphinx. Both
that and the Documentation/DocBook/Makefile are now expected to handle
all the documentation targets, explicitly ignoring them if they're not
relevant for that particular toolchain. The changes to the existing
DocBook Makefile are kept minimal.

There is graceful handling of missing Sphinx and rst2pdf (which is
needed for pdf output) by checking for the tool and python module,
respectively, with informative messages to the user.

If the Read the Docs theme (sphinx_rtd_theme) is available, use it, but
otherwise gracefully fall back to the Sphinx default theme, with an
informative message to the user, and slightly less pretty HTML output.

Sphinx can now handle htmldocs, pdfdocs (if rst2pdf is available),
epubdocs and xmldocs targets. The output documents are written into per
output type subdirectories under Documentation/output.

Finally, you can pass options to sphinx-build using the SPHINXBUILD make
variable. For example, 'make SPHINXOPTS=-v htmldocs' for more verbose
output from Sphinx.

This is based on the original work by Jonathan Corbet, but he probably
wouldn't recognize this as his own anymore.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>