replace dockers/MathJaxFonts with KaTeX/katex-fonts submodule (#891)

CONTRIBUTING.md

@@ -131,6 +131,29 @@ In general, try to make your code blend in with the surrounding code.
  - commits should be squashed before merging
  - large pull requests should be broken into separate pull requests (or multiple logically cohesive commits), if possible
 
+ ## Working with submodules
+
+The fonts for KaTeX live in a submodule which appears in submodules/katex-fonts.
+Most of the time you won't have to worry about this unless you're making
+changes to fonts or switching between branches where submodules/katex0fonts
+point to different commits.
+
+If you're not familiar with submodule, it's probably easiest to get started by
+adding the following aliases to your .gitconfig:
+```
+[alias]
+  # Versions of commands that handle submodules properly.
+  co = "!f() { git checkout \"$@\" && git submodule update --init --recursive; }; f"
+  p = "!f() { git pull \"$@\" && git submodule update --init --recursive; }; f"
+  m = "!f() { git merge \"$@\" && git submodule update --init --recursive; }; f"
+  gsu = "!f() { git submodule sync --recursive && git submodule update --init --recursive; }; f"
+```
+`git co`, `git p`, and `git m` work just like `git checkout`, `git pull`, and
+`git merge` respectively but automatically update submodules.  For more info
+about how to use git submodules see https://chrisjean.com/git-submodules-adding-using-removing-and-updating/.
+
+When submitting pull requests, that update katex-fonts, you'll need to submit
+two pull requests: one for [KaTeX/katex-fonts](https:/github.com/KaTeX/katex-fonts) and one for [Khan/KaTeX](https://github.com/Khan/KaTeX).
 ## CLA
 
 In order to contribute to KaTeX, you must first sign the CLA, found at www.khanacademy.org/r/cla