relocated content to pages dir

docs/README.md

docs/_Sidebar.md

docs/pages/About-the-NSHMP.md

docs/pages/Building-&-Running.md

docs/pages/Calculation-Configuration.md

docs/pages/Developer-Basics.md

docs/pages/Functional-PSHA.md

docs/pages/Ground-Motion-Models.md

docs/pages/Hazard-Model.md

docs/pages/Logic-Trees-&-Uncertainty.md

docs/pages/Magnitude-Frequency-Distributions.md

docs/pages/Model Editions.md

docs/pages/Model-Files.md

docs/pages/Model-Structure.md

docs/pages/Rupture-Scaling-Relations.md

docs/pages/Site-Specification.md

docs/pages/Source-Types.md

docs/pages/USGS-Models.md

docs/pages/images/psha-formula.png

docs/pages/images/psha-functional.png