Allows lazy-loaded (i.e. data-src) iframes to be preloaded when they
come within the viewDistance, rather than once they're visible.

Resolves #2211

The Install was waaaay at the bottom, which didn't make sense to scroll through all the detailed documentation, just to find how to get started with an install.

Fixed broken link to markdown options doc

- Use `single backticks` consistently for inline code snippets.
- Add double linefeeds before level 2 headings.
- Add a single linefeed after each heading.
- Minor docs edits.

Ref. #1856, #2006, #2045.

This is a more open approach to allow customisation of all MathJax options instead of select options only.