Run fstardoc (with some modifications) over standard library

[mgritter]

This PR refreshes the standard library documentation using fstardoc.

Instead of using pandoc (and writing a syntax highlighting grammar in the format it understands) my revised script uses the GitHub service to render markdown -> html, so we get the syntax highlighting that GitHub performs.

The script used to generate this is a work in progress at
https://github.com/mgritter/FStar/blob/mgritter/script_html_conversion/.scripts/fstardoc/generate-ulib.py

Additions:

• Index page includes the first sentence in the module documentation, if it can be found.
• Instead of showing the code for open or module X = Y or include, the document translates those dependencies into a bulleted list with hyperlinks.
• Language annotation on code blocks is FStar not fstar

Diffs to fstardoc can be seen here: FStarLang/FStar@master...mgritter:mgritter/script_html_conversion

[ghost]

All CLA requirements met.

[mgritter]

I broke the CSS for syntax highlighting... will fix.