Signup section should render Grav forms

[hughbris]

The markup in the demo and source template will submit a form to a target that could be configured to handle the request appropriately, but it's also sensible to support a Grav form that has been declared in frontmatter.

Extend the partials/sections/signup.html.twig template to load form definitions from page frontmatter.

Then document how to tweak this as per #6.

Not that this is not related to the Signup button form the demo and source template, though that may link through to the same form handler.