Begin rework of documentation

Remove old documentation files and re-implement:

 - `index.rst`
 - `introduction.rst`
 - `installation.rst`
Write introduction

First draft of introduction in Sphinx documentation.

[ci skip]
New file to store GL2007 example

[ci skip]
Complete first draft of introduction

[ci skip]
Early work on GL2007 Model SIM example

[ci skip]