docs: update docker install section

[fyliu]

Fixes #645

What changes did you make?

• mainly adding note to do post-install steps
• see before and after descriptions for other improvements

Why did you make the changes (we will use this info to test)?

• the post-install is needed for running docker as the normal user
• see before and after descriptions for others

Screenshots of Proposed Changes

before	after
- docker compose not needed anymore, since it's part of docker now. - installation link target is really small.	- added short description of what it does and how it benefits us - larger installation link - added note about post-install
- complicated docker vs. docker desktop instructions - systemctl command is not portable across Linuxes. - step description the same as the command - long step that should be split up - it' s really 2 things	- simplified step that works for both docker and docker desktop - complexity hidden behind the + sign - simplified description - split long step into 2 parts, with better quoting
	- content that's shown when clicking on the + - explains what the command should do, and the common failure case and solution that includes both docker and docker desktop.