Fix broken equations in docs (#991)

Switches back to MathJax. Closes #988.

Author: mlubin

docs/make.jl

@@ -2,8 +2,11 @@ using Documenter, MathOptInterface
 
 makedocs(
     sitename = "MathOptInterface",
-    # See https://github.com/JuliaDocs/Documenter.jl/issues/868
-    format = Documenter.HTML(prettyurls = get(ENV, "CI", nothing) == "true"),
+    format = Documenter.HTML(
+        # See https://github.com/JuliaDocs/Documenter.jl/issues/868
+        prettyurls = get(ENV, "CI", nothing) == "true",
+        mathengine = Documenter.MathJax()
+    ),
     # See https://github.com/JuliaOpt/JuMP.jl/issues/1576
     strict = true,
     pages = [