Fixed Sphinx build warnings

Currently Sphinx emits:

```
mathjax-docs/index.rst:29: WARNING: toctree contains reference to nonexisting document 'match-web-fonts'
mathjax-docs/mathml.rst:130: WARNING: Title underline too short.
mathjax-docs/platforms/movable-type.rst:: WARNING: document isn't included in any toctree
mathjax-docs/platforms/wordpress.rst:: WARNING: document isn't included in any toctree
```

This fixes all these issues.