* Add 18.04 install guide and and move 16.04 guide
Adds 18.04 install guides to "Installing LibreNMS" page.
Moves 16.04 install guides to "Old install docs" section.
* Update index.md
Update index tree to 1804 over 1604
* docs: Continue to reorganize things
Make landing pages for /Installation, /API, etc.
Split API out into multiple docs
Properly use multi-level navigation
Add lower nav buttons (tops are bug so disable for now)
Add logo for fanciness
Refine index page a bit
* Improve API TOC layout
* Couple of fixes.
* Fix API/API-Docs, assumed it was at the right url.