KaTeX website & documentation (#1484)

* website/docs: initial commit

* Change secondaryColor

* Fix index.css not being copied and included on global stylesheet

* Fix stylesheet link

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* Change documentation link to API(Usage)

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* Add `Libraries` in usage

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* Remove documentation from `README.md` and add link to the site

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* Use KaTeX in the parent directory to build Markdown

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* Revise function support page. Avoid error msgs.

* General edit to function support page

docs/issues.md

@@ -0,0 +1,20 @@
+---
+id: issues
+title: Common Issues
+---
+- Many Markdown preprocessors, such as the one that Jekyll and GitHub Pages use,
+  have a "smart quotes" feature.  This changes `'` to `’` which is an issue for
+  math containing primes, e.g. `f'`.  This can be worked around by defining a
+  single character macro which changes them back, e.g. `{"’", "'"}`.
+- KaTeX follows LaTeX's rendering of `aligned` and `matrix` environments unlike
+  MathJax.  When displaying fractions one above another in these vertical
+  layouts there may not be enough space between rows for people who are used to
+  MathJax's rendering.  The distance between rows can be adjusted by using
+  `\\[0.1em]` instead of the standard line separator distance.
+- KaTeX does not support the `align` environment because LaTeX doesn't support
+  `align` in math mode.  The `aligned` environment offers the same functionality
+  but in math mode, so use that instead or define a macro that maps `align` to
+  `aligned`.
+- MathJax defines `\color` to be like `\textcolor` by default; set KaTeX's
+  `colorIsTextColor` option to `true` for this behavior.  KaTeX's default
+  behavior matches MathJax with its `color.js` extension enabled.