[TIDOC-1847] Added multi-level TOC to cloud guide pages.

Number of <hN> tags that are included in TOC is determined
by `max_level` property.
Added JSDuck --guide-toc-level JSDuck command-line option.

Defaults to 2, which is the same as the current TOC level.

```
jsduck ... --guide-toc-level 2
```
Changed TOC to use nested <ul>s instead of numbered lines with line breaks.
Fixed issue with TOC generation of documents with non-consecutively increasing headings, e.g. h1, h4, h6

TOC list indentation now properly reflects actual heading sequence
Merge pull request #50 from tstatler/TIDOC-1847

[TIDOC-1847] Added multi-level TOC to cloud guide pages.