Document the `templates.default.layoutFile` config property

[hegemonic]

In JSDoc 3.3.0, the config file may contain a templates.default.layoutFile property, which specifies a relative or absolute path to a file that overrides the default template's layout.tmpl file.

Relative paths are resolved against the path to the config file; the current working directory; and the JSDoc directory, in that order.

See jsdoc/jsdoc#480.

[hegemonic]

Fixed on master.