Adding documentation for db_config file

[nszchagas]

What was done

• Included documentation for database configuration file structure.

[trasher]

Database configuration file is automatically generated; there is no need to document it.

Some part of that file are automatically calculated; this is really not a good idea to try changing this file.

I'd vote against the integration of this PR.

[nszchagas]

Even though it's automatically generated, it's important to know where it is stored.

I'm running GLPI on a Docker container using a personalized docker image, and there's the need to bind this configuration file on my server, in order to protect my database variables, since the image is shared in containers registries.

[trasher]

Path is already present in the advanced installation part, but its does not use the constant - that must be changed to be consistent across the doc.

Path is quite important, yes, so just keep that information, and remove file contents.