Change content of new HTML files to not be empty.

[K-LV]

Right now, when you create a new HTML document, the content of that new file is empty. This leads to problems when trying to preview it. This solution is to intercept the writing process of HTML files and change the contents to a default page.

[humphd]

Make sure we put documentation about why we're doing this (e.g., because we always preview docs immediately in LiveDev) in the code that does this.

[gideonthomas]

Fixed with 155a862