document GitHub Actions CI

[cfm]

Describe the change

As of freedomofpress/securedrop#7223, we have moved entirely from CircleCI to GitHub Actions. We should take this opportunity to document the current CI workflows while they're still fresh.

User Research Evidence

By team request.

Additional context

• 152

• https://wiki.freedom.press/wiki/CI,_CD_for_SecureDrop
• https://wiki.freedom.press/wiki/Demo_instance

[legoktm]

I've posted a first pass at https://github.com/freedomofpress/securedrop-dev-docs/pull/183, there's still more to do re: docs and CD. I tried to strike a balance between being useful and also not being overly specific that requires updates every time we make small change.

Regarding the internal wiki pages:

• https://wiki.freedom.press/wiki/CI,_CD_for_SecureDrop

Nothing super useful here, will blank it and point it to the dev docs repo once my PR is merged.

• https://wiki.freedom.press/wiki/Demo_instance

This is great, I'm going to rst-ify it and copy it basically wholesale to this repo.