Batch of improvements for the manual installation documentation:

* add prerequisites section
* add shell server section
* add keepproxy section
* many other updates throughout

refs #4186
Keepproxy just needs an anonymous token.

refs #4186
Fix typo.

refs #4186
Add keepstore installation page.

refs #4186
Merge branch 'master' into 4186-install-doc-improvements-2
Clean up the installation overview pages a bit more.

refs #4186
Merge branch '4186-install-doc-improvements-2'

refs #4186