docs: pdf: add all Documentation/*/index.rst to PDF output

Currently, all index files should be manually added to the
latex_documents array at conf.py.

While this allows fine-tuning some LaTeX specific things, like
the name of the output file and the name of the document, it
is not uncommon to forget adding new documents there.

So, add a logic that will seek for all Documentation/*/index.rst.
If the index is not yet at latex_documents, it includes using
a reasonable default.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/conf.py

@@ -410,6 +410,21 @@ latex_documents = [
      'The kernel development community', 'manual'),
 ]
 
+# Add all other index files from Documentation/ subdirectories
+for fn in os.listdir('.'):
+    doc = os.path.join(fn, "index")
+    if os.path.exists(doc + ".rst"):
+        has = False
+        for l in latex_documents:
+            if l[0] == doc:
+                has = True
+                break
+        if not has:
+            latex_documents.append((doc, fn + '.tex',
+                                    'Linux %s Documentation' % fn.capitalize(),
+                                    'The kernel development community',
+                                    'manual'))
+
 # The name of an image file (relative to this directory) to place at the top of
 # the title page.
 #latex_logo = None