Update documentation for synbiohub2 docker install

[cjmyers]

The documentation for the synbiohub2-docker install should make it clear how to install release versus snapshot.

It should also explain how to update the backend/backendSS variables. (NOTE: should figure out if we can do this better on the command line without editing the file)

It should also point to documentation about setup.

Finally, it would be nice if included some instructions about caddy and systemctl setup. Granted there are other ways to do this, but it is nice to provide at least one solution, and it is good docs for us.

[danielfang97]

SynBioHub3 README should point to here as well

[cjmyers]

Also update the setup wiki

[cl117]

code: https://github.com/SynBioHub/synbiohub2-mkdocs
wiki: https://wiki2.synbiohub.org/

[PeterHindes]

I think we should get rid of the release branch to avoid confusion and rename snapshot to main

[cjmyers]

Let's keep release branch. There is a reason for this, as this affects how select which docker build to use.

[PeterHindes]

SynBioHub/synbiohub2-docker#25
this pull request adds support for setting backend url from the cli without editing the dockercompose file as requested and makes backendss valid in all cases

and SynBioHub/synbiohub2-mkdocs#9
adds info on it to the official docs