Add template tag and documentation

This templatetag acts like the CMS's templatetag 'render_model_block' but with
a plugin instead of a model. This is used to link from a block of markup to a
plugin's changeform.

This is useful for UIs that have some plugins hidden from display in preview
mode, but the CMS author needs to expose a way to edit them anyway. It is also
useful for just making duplicate or alternate means of triggering the change
form for a plugin.

This would typically be used inside a parent-plugins render template. In this
example code below, there is a parent container plugin which renders a list of
child plugins inside a NAV block, then the actual plugin contents inside a
DIV .contentgroup-items block. In this example, the nav block is always shown,
but the items are only shown once the corresponding navigation element is
clicked. Adding this render_plugin_block makes it significantly more intuitive
to edit a child plugins content, by double-clicking its nav item in edit mode.