Fix #3742: missing `Index` bookmark and TOC entry in PDF builds at RTD

The trick is to create an Index TOC/bookmarks entry in anticipation of
makeindex execution, as RTD (currently) does only one pdflatex run after
makeindex.

    https://github.com/rtfd/readthedocs.org/issues/2857

The workaround works for Sphinx own docs, which uses ``'manual'``
docclass and puts the general index at very end of document.

doc/conf.py

@@ -56,7 +56,12 @@ latex_logo = '_static/sphinx.png'
 latex_elements = {
     'fontpkg': '\\usepackage{palatino}',
     'passoptionstopackages': '\\PassOptionsToPackage{svgnames}{xcolor}',
-    'printindex': '\\footnotesize\\raggedright\\printindex',
+# fix missing index entry due to RTD doing only once pdflatex after makeindex
+    'printindex': r'''
+\IfFileExists{\jobname.ind}
+             {\footnotesize\raggedright\printindex}
+             {\begin{sphinxtheindex}\end{sphinxtheindex}}
+''',
 }
 latex_show_urls = 'footnote'