diagram installation, configuration, update, and backup flows

[cfm]

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 freedomofpress/securedrop#6826 (comment), @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).