Add documentation for [:vspace](1em)

[kno10]

This is a convenient macro to insert spacing, yet it is not mentioned in the documentation.

It would be good if the clear:both would be optional. I don't think a vspace implies a float barrier.

[monofon]

You might want to build your own vspace with a template macro:

---
templates:
  vspace: |
    <div style="display:block; height::(url);"></div>
title: Custom vspace macro
---

Use it

[@vspace](10em)

like this.