Cherry-pick new commits from master branch:

* Correct some typoes
* Update installation part
  1) remove manual way installation and appendix with vm configuration for developer guide (temprory)
  2) Add css for developer guide
  3) add link to additional linux repos in installation guide
* Add folder with style files to git
* Update installation commands
* Add installation from venv chapter
* Update ADMIN guide
* Typos and style fixes.
* Move installation chapter to admin guide

Change-Id: I7c33a55de13a5974f41676ee9a77010d73a12050