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

[michalkleiner]

Thanks @bomoko. One more question comes to mind that is not mentioned in the addition to the docs — do you need to manually create e.g. symlinks from the absolute path to the relative path, or does Matomo do that for you? Either way it would be good to have that mentioned (if it works itself or if it needs to be done by the person configuring/installing Matomo).