Flesh out documentation

[srid]

The docs are more of a skeleton right now. Flesh it out further. Should be beginner-friendly.

• [ ] #136
• [x] #276
• [ ] #212
• [ ] https://github.com/srid/haskell-flake/issues/245
• [ ] #394
• [ ] ...

[shivaraj-bh]

Additional points to include in Overriding dependency:

• Include other common settings options like jailbreak and broken.
• A section talking about default settings for all the packages.