Template for new files at GitHub: add info on customization

[deining]

The template file new-page-template.md specifies the frontmatter and content used for new pages at GitHub (via link Create new child page in the right sidebar). This PR adds documentation on how to override the frontmatter and the content defined in this file via a project specific file in your project.

[chalin]

I think that we should document this feature in the User Guide instead of adding an HTML comment to the template file, since the information is more pertinent to website maintainers than to contributors adding a new page.

My 2 cents. WDYT @emckean @LisaFC @geriom?

[deining]

I think that we should document this feature in the User Guide

This is documented in the user guide already:

* Create child page: Brings the user to a create new file form in their fork of your docs repo.
The new file will be located as a child of the page they clicked the link on.
The form will be pre-populated with a template the user can edit to create their page.
You can change this by adding assets/stubs/new-page-template.md to your own project.

I think that we should document this feature in the User Guide instead of adding an HTML comment to the template file

The HTML comment is an additional reminder, thought as backing of the docu in the userguide. We had support questions in the past regarding the use of this file, so think adding the comment makes sense to me.