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.

Closes #2005.

Closes #2075.

Closes #1808.

Add an option to display advice on whether the current pace of the
presentation is on track for the right timing (shown as green), and if
not, whether the presenter should speed up (shown as red) or has the
luxury of slowing down (blue).

The pacing timer can be enabled by configuring by the `defaultTiming`
parameter in the `Reveal` configuration block, which specifies the
number of seconds per slide.  120 can be a reasonable rule of thumb.
Timings can also be given per slide `<section>` by setting the
`data-timing` attribute.  Both values are in numbers of seconds.

When the option is enabled, clicking on the timers will reset the timer
to the beginning of the current slide, i.e. as if pacing was perfectly
on track, not to zero as if the presentation had just begun.

This can be used by plugins which are concerned with helping the
presenter with their pace.