Update index.md and add pgadmin.md to docs/sandbox/apps/

[RaneyDazed]

adding pgadmin to docs. Just thought it would be helpful if anyone wanted to look it up.

[RaneyDazed]

I'm having a brain fart, how the hell do I specify which little category its in on the drop down menu on the left side of the page?

[owine]

Inside mkdocs.yml