User guide outline

[GSStnb]

Revised User Guide index and initial Drawing Setup guide

[lordofbikes]

Many thanks Gary!

One point was confusing me on review, the rename of layers.rst to layer.rst.
It's not an issue, Sphinx handle this correct, as inside the file the site is named layers and so everything works fine. But it took a moment to understand why it is working.
Perhaps you can revert this rename to make it more consistent.