doc: Add manual for the config file.

[bentley]

Still very, very rough, but I’m looking for feedback.

• The title only makes sense if config.ini gets renamed to mgba.ini.
• Is there a complete list of settings?
• How many of these are qt only? They should go in the qt section.
• Are there any other sections besides [ports.qt] and inputs? What’s the difference between gba.input-profile and gba.input?
• Default values should probably be documented for most of these.

[endrift]

fwiw I'm possibly going to be doing a revamp where I convert to YAML at some point, at which point I can easily rename the config file to mgba.yaml.

[lyneca]

Just like to bump this, is there a complete list of settings available?

[endrift]

Not at the moment. They're kinda added without much thought so such a list would rapidly become obsolete.