Clarify D2::Template::Simple's role in life

[cromedome]

This is one thing to come out of #1615. Got me to thinking about why we have two template systems in core when we really want to steer devs to other template engines.

After talking with @SysPete and @veryrusty, it was decided we should update the pod here to clarify what role Dancer2::Template::Simple fills, and to consider splitting Dancer2::Template::Tiny to another distribution.

[ghost]

@cromedome This sounds very good! I did some work in #1616 to try and remove Template::Simple, but I keep running in to issues with generating a new application and using Template::Tiny to create the config and app.psgi files from skeleton.
It is probably a better way forward to clarify the docs and the try removing the forked version of Template::Tiny instead.

[bigpresh]

Approved - although I think yanking it out of core entirely into its own CPAN dist might not be a bad thing. D1 users migrating to D2 who used to use Dancer::Template::Simple would have to then go install it, but I don't think many D1 users used D::T::Simple - I think many would have switched it out for a more full-featured engine, and as long as we document clearly on the migration doco that you'll need to install it if your app used D::T::Simple, that sounds fair to me.

[cromedome]

Merged, thanks!