docs: add make docs target to generate inheritance/call graphs via Doxygen

[cfm]

Description

Adds a make docs target to generate Doxygen documentation from Python sources and docstrings. After #1452 (comment), my particular interest here is in its class-inheritance and function-call graphs.

User story: As a developer, I want to be able to see diagrams of the call/caller relationships among functions and the inheritance relationships among classes, so that I can explore (and observe architectural smells in) the codebase visually.

If we ever were to publish this kind of documentation—for example, for securedrop-sdk—we'd probably go with Sphinx as the default in the Python ecosystem. I went with Doxygen here because it has built-in support for generating (the Graphviz source for) these graphs, whereas Sphinx requires an extension like Pyan. For an optional development tool, my weak preference is for (a) apt install doxygen graphviz over (b) pip install sphinx pyan3 which depends on apt install graphviz anyway.

Test Plan

On this branch:

$ make docs
doxygen(1) is not available in your $PATH.  Is it installed?
make: *** [Makefile:195: docs] Error 1

• make docs fails gracefully without Doxygen installed.
• Install Doxygen: sudo apt install doxygen

$ make docs
Graphviz's dot(1) is not available in your $PATH.  Is it installed?
make: *** [Makefile:196: docs] Error 1

• make docs fails gracefully without Graphviz installed.
• Install Graphviz: sudo apt install graphviz

$ make docs
[...]
finished...
Now open "/home/user/securedrop-client/docs/html/index.html" in your browser.

• make docs succeeds, including underlying dot invocations.
• The named file opens in your browser.
• Classes → Classes List is full of classes.
• securedrop_client → api_jobs → base → ApiJob shows:
  • an inheritance diagram for ApiJob subclasses; and
  • a collaboration diagram for ApiJob → QueueJob → QObject.

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs or network (via the RPC service) traffic, Qubes testing in the staging environment is required. For fine tuning of the graphical user interface, testing in any environment in Qubes is required. Please check as applicable:

• I have tested these changes in the appropriate Qubes environment
• I do not have an appropriate Qubes OS workstation set up (the reviewer will need to test these changes)
• These changes should not need testing in Qubes

If these changes add or remove files other than client code, the AppArmor profile may need to be updated. Please check as applicable:

• I have updated the AppArmor profile
• No update to the AppArmor profile is required for these changes
• I don't know and would appreciate guidance

If these changes modify the database schema, you should include a database migration. Please check as applicable:

• I have written a migration and upgraded a test database based on main and confirmed that the migration applies cleanly
• I have written a migration but have not upgraded a test database based on main and would like the reviewer to do so
• I need help writing a database migration
• No database schema changes are needed

[eaon]

LGTM! Graphs are 🆒