doc: document user invitations & how to disable them

Created by: slimsag

Adds the docs that we need for user invitations (what they are and how to disable them). @philipp-spiess can you link these from the frontend?

How it looks: https://user-images.githubusercontent.com/3173176/157572667-495491c6-8406-402f-88e1-a1bc3f6db016.png

Signed-off-by: Stephen Gutekanst [email protected]

Test plan

sg run docsite and manually checking the page.