First pass on Installation Guide

docs/backdrop/index.md

docs/drupal7/index.md

docs/drupal8/index.md

docs/general/i18n_l10n.md

docs/general/permissions.md

docs/general/requirements.md

docs/general/troubleshooting.md

docs/images/backtrace.png

docs/images/clean-up-caches.png

docs/images/fatal-error.jpg

docs/images/wp-permalinks.png

docs/joomla/index.md

docs/planning/cms.md

docs/planning/demo.md

docs/planning/hosting.md

docs/planning/index.md

docs/planning/transition.md

docs/wordpress/index.md

mkdocs.yml