Various features of the skin should be optional

[bertysentry]

Enhancement

The skin comes with a bunch of nice features, but sometimes they "get in the way" of the user, or they are superfluous for an existing documentation site that uses other techniques to achieve the same result.

Specification

• Update tools.vm and skin.vm to activate all of the extra-features if they are not disabled under <custom> in site.xml
• Document all variables, both in settings.md and each corresponding feature

List of configurable features:

Feature	Variable
Parsing of Maven properties	<parseMavenProperties>
Produce search index	<buildSearchIndex>
Autozoom of images	<autoZoom>
Conversion of images to WEBP	<webp>
Code syntax highlighting	<highlightCode>
Fix headings	<fixHeadings>