[docs] Document non-edge installation using Docker Compose

[c-gabri]

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [ ] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [x] I have updated the documentation.

Reference to Existing Issue

Closes #516.

Description of Changes

Add documentation for installing any release (not just edge) with the Docker Compose method.

[c-gabri]

Maybe there should also be documentation on how to upgrade using the Docker Compose method, either in this PR or in a new one. Let me know.