Adds matomo plugin directory env documentation

[bomoko]

Description:

This PR updates the developer documentation around the environment variables MATOMO_PLUGIN_DIRS and MATOMO_PLUGIN_COPY_DIR - two variables particularly helpful for containerized installations, but which seem to currently be underdocumented.

Closes #835

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed