docs: Move relevant SSO documentation to stackspin/stackspin

Closes #1357 (closed)

I checked the documents in:

https://open.greenhost.net/stackspin/single-sign-on/-/tree/main/docs

• Added architecture.rst to this repo as "reference/sso_architecture"
• Ignored development.md because it is outdated and superseeded by the README in dashboard-backend
• Ignored helmchart.md because it is about a helmchart we're not using anymore
• Ignored api-examples.sh because it is outdated, people can look at the dashboard-backend repo for A