Apply a default layout for the MarkBind website

[damithc]

Suggestion: use a default layout for the MarkBind website

[damithc]

@Chng-Zhi-Xuan I tried to do this while doing #518 but couldn't get the SiteNav to render properly.

[Chng-Zhi-Xuan]

Update:

@damithc , the issue resides within Page.prototype.prepareTemplateData. If a siteNav is not explicitly declared within the frontmatter, the associated CSS files will not be inserted into the page.

Will need to modify the logic to check for siteNav within layouts as well.

[damithc]

Raising priority as this seems to be a problem in the layouts feature.