sphinx: enable intersphinx extension

This extension can generate automatic links to the documentation of
objects in other projects. We will use it to use cross references with
the Bibtake manual, for example.

(From yocto-docs rev: 5add9854b112f93acba982f237fbfa83aee80d77)

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/conf.py

@@ -35,6 +35,7 @@ sys.path.insert(0, os.path.abspath('sphinx'))
 extensions = [
     'sphinx.ext.autosectionlabel',
     'sphinx.ext.extlinks',
+    'sphinx.ext.intersphinx',
     'yocto-vars'
 ]
 autosectionlabel_prefix_document = True
@@ -72,6 +73,11 @@ extlinks = {
     'oe_lists': ('https://lists.openembedded.org%s', None),
 }
 
+# Intersphinx config to use cross reference with Bitbake user manual
+intersphinx_mapping = {
+    'bitbake': ('https://docs.yoctoproject.org/bitbake/', None)
+}
+
 # -- Options for HTML output -------------------------------------------------
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for