$ formatting documentation markup

[sn6uv]

This issue was raised by @bnjones in #341.

The mathics docs for the documentation markup spec:

$identifier$
    a variable identifier in Mathics code or in text.

It would be useful to extend this to allow more things. The example raised in #341 was for the less or equal symbol. Ideally we'd like \le for the pdf docs, ≤ for the web interface docs and also <= (or maybe ≤) for the plaintext docs (useful for introspection by e.g. Information).

This also applies to Gamma, Superscript etc.

Proposed solutions:

• Support more TeX code between $ symbols.
• Use mathics expressions and format them as MathML, TexFom etc.

[rocky]

Another option: support ReStructuredText and/or Sphinx the docstring markup and use exisitng Python tools to render and collect into documentation.