Consolidate test plans on `securedrop` wiki; phase out other GitHub wikis

[eloquence]

We have three GitHub wikis:
https://github.com/freedomofpress/securedrop/wiki
https://github.com/freedomofpress/securedrop-client/wiki
https://github.com/freedomofpress/securedrop-workstation/wiki

They're mainly used for updating test plans. Historically, they were more active to store meeting notes and architecture docs, but these days, such documents mainly live in:

• The dev docs
• GitHub issues
• Discussions (also centralized on securedrop)
• FPF's internal wiki
• Assorted GDocs

I would propose:

• Move test plans to securedrop wiki
• Review other materials for freshness; preserve useful other docs in https://developers.securedrop.org/ or securedrop wiki
• Close securedrop-workstation and securedrop-client wikis

This is to reduce the number of places we have to look for things, or that we consider when creating new docs; generally, the dev docs seem like the best place for tips & architecture documentation.

[eloquence]

I'll note that while it's nice that GitHub renders Mermaid syntax, this example (note the "Unsupported markdown: list") suggests that it's not necessarily awesome for long-lived documents. With our dev docs, we can be sure to always stash both the image and the source.