AGG link changed. Also fix other doc warnings

Here are some of the remaining warnings I see (I have a total of 77):

```
...14: WARNING: undefined label: mpl-color-spec (if the link has no caption the label must precede a section header)
<autosummary>:None: WARNING: toctree contains reference to nonexisting document u'api/matplotlib.tri.Triangulation.edges'
<autosummary>:None: WARNING: toctree contains reference to nonexisting document u'api/matplotlib.tri.Triangulation.neighbors'
<autosummary>:None: WARNING: toctree contains reference to nonexisting document u'api/matplotlib.tri.TriAnalyzer.scale_factors'
None:None: WARNING: toctree contains reference to nonexisting document u'api/matplotlib.tri.Triangulation.edges'
None:None: WARNING: toctree contains reference to nonexisting document u'api/matplotlib.tri.Triangulation.neighbors'
None:None: WARNING: toctree contains reference to nonexisting document u'api/matplotlib.tri.TriAnalyzer.scale_factors'
/home/ben/Programs/matplotlib/doc/users/screenshots.rst:297: WARNING: undefined label: user_interfaces-embedding_in_qt (if the link has no caption the label must precede a section header)
/home/ben/Programs/matplotlib/doc/users/whats_new.rst:79: WARNING: undefined label: ~examples/statistics/boxplot_demo.py (if the link has no caption the label must precede a section header)
/home/ben/Programs/matplotlib/doc/users/whats_new.rst:79: WARNING: undefined label: ~examples/statistics/bxp_demo.py (if the link has no caption the label must precede a section header)
/home/ben/Programs/matplotlib/doc/users/whats_new.rst:1079: WARNING: undefined label: legend-complex-plots (if the link has no caption the label must precede a section header)
```
I am also seeing that some rst files are not getting included into the TOC. In particular, the patheffects guide.
One thing to remember with sphinx is that it only emits warnings and errors for files that it is updating. So one must remember to do a fresh doc build from time to time to catch things that have gone out of date or were broken, but never fixed.