diagram installation, configuration, update, and backup flows

Describe the change

Because our various provisioning flows (a) are unusually complicated and (b) differ subtly from one another, we should diagram them precisely.

How will this impact users?

As a contributor, I'd like to understand the different workflows I may encounter in different environments, so that I can execute and troubleshoot them more effectively.

As a maintainer, I'd like to have a high-level view of the different workflows, so that I can reason about them and any changes we may make to them.

User Research Evidence

In https://github.com/freedomofpress/securedrop/pull/6826#issuecomment-1579102111, @nathandyer and I used the following sequence diagram to illustrate the flow of values and artifacts in a securedrop-admin install run:

Additional context

#8 proposes something similar at the repository level, which I imagine would include a graph (in "space"). This should be a set of task-specific sequence diagrams (in time).

Start with the existing Mermaid sequence diagram linked from the PR 6826 comment, then compare it with the installation, configuration, update, and backup provisioning flows described in this issue. Review issue #8 for repository-level context. Done means task-specific sequence diagrams clearly document each workflow and its environment-dependent differences.