docs: introduce product tests

[AlexanderLanin]

Improve High-Level Testing for Documentation

Problem Statement

While we have tests for individual components like extensions, we lack comprehensive high-level tests for our documentation system as a whole. This means integration issues may go unnoticed.

Missing Coverage

• Does PlantUML render correctly in live_preview mode?
• Is the source code linker triggered in incremental mode?

Scope of the Test

The goal is to validate the full integration of our documentation setup, ensuring that everything works together correctly. This includes:

• Bazel Integration: Ensuring our .bzl files behave as expected.
• Python Launcher Scripts: Verifying that our custom scripts work correctly.
• Sphinx and Extensions: Making sure Sphinx and our custom extensions function properly.

Test Approach

• The tests should replicate real user workflows as closely as possible.
• They should use the exact commands a user would run, such as bazel run //docs:incremental