Improve automating Shlink installation outside of Docker

[acelaya]

Right now Shlink covers pretty well two "installation" use cases.

• Interactive one, outside of docker, where the installation tool covers requesting and generating config options, initializing the database, etc.
• Unatended one with docker, where the entry point covers initializing database, clearing caches, etc.

Both of the approaches above make use of some "hidden" commands to initialize the database.

It is also documented that you can provide config to Shlink via environment variables, even when not using Docker. However, this use case lacks a way to initialize the database and other stuff.

Provide a mechanism to do this, in the form of some built-in tool, but also documentation covering this use case.