* 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.
I only really ordered the top level nav and installation, others could be customized if desired.
Shorten the header to LibreNMS Docs, so the menu stays nice on smaller screens, maybe even should shorten it to LibreNMS
Add some links on the index.md page