fix(presets): add --mathml to pandoc markdown args (#53)

* fix(presets): add `--mathml` to `markdown` and `github` pandoc args

Problem: pandoc's default HTML math renderer cannot handle most TeX and
dumps raw LaTeX source into the output. `--mathjax` and `--katex` are
incompatible with `--embed-resources` because pandoc cannot inline
dynamically-loaded JavaScript modules and fonts.

Solution: add `--mathml` to both `markdown` and `github` preset args.
MathML is rendered natively by all modern browsers with no external
dependencies, making it the only math option compatible with
self-contained HTML output.

* docs(presets): add math rendering section with KaTeX recipe

Problem: the `markdown` and `github` presets now default to `--mathml`
but users may want KaTeX or MathJax rendering instead, and the
incompatibility with `--embed-resources` is non-obvious.

Solution: add a `preview-math` section to the presets docs explaining
the default, why `--katex`/`--mathjax` require dropping
`--embed-resources`, and a concrete recipe for KaTeX with `github`.

* test(presets): update `markdown` and `github` args assertions for `--mathml`

lua/preview/presets.lua

@@ -224,7 +224,7 @@ M.markdown = {
   ft = 'markdown',
   cmd = { 'pandoc' },
   args = function(ctx)
-    return { ctx.file, '-s', '--embed-resources', '-o', ctx.output }
+    return { ctx.file, '-s', '--embed-resources', '--mathml', '-o', ctx.output }
   end,
   output = function(ctx)
     return (ctx.file:gsub('%.md$', '.html'))
@@ -250,6 +250,7 @@ M.github = {
       ctx.file,
       '-s',
       '--embed-resources',
+      '--mathml',
       '--css',
       'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/dist/gfm.css',
       '-o',