Create nav_footer_custom (#474)

**Rationale**
I would like to customize the site footer content without needing to override the entire default layout. Ideally, I'd like to shorten the attribution to something like "Built with Just the Docs"

**Implementation**
Create a new `site_footer.html` include that users can override.

**Possible changes**
I debated whether or not the `<footer class="site-footer">` should be in/out of the include. I opted for placing it inside the include so I have the option of disabling the footer entirely by creating an empty site_footer file in my site.

Co-authored-by: Matt Wang <matt@matthewwang.me>
Co-authored-by: Patrick Marsceill <pmarsceill@users.noreply.github.com>