Restructure FAQ section

Problem statement

Currently the FAQ sections on nix.dev and the NixOS Wiki are an unstructured collection of question-answer pairs. Some of them are for troubleshooting, some of them are actual recipes for performing common tasks, some are conceptual in nature.

Objective

Since many answers are not as helpful as they could be, we will sort through all questions, and for each make a PR that updates the answer and adds it to the appropriate category on nix.dev.

Tasks

• Check each question on NixOS Wiki: FAQ
• Make a PR for each question on nix.dev below

Troubleshooting

• What to do if a binary cache is down or unreachable?
• How do I fix: error: querying path in database: database disk image is malformed
• How do I fix: error: current Nix store schema is version 10, but I only support 7
• I’m getting: writing to file: Connection reset by peer

Recipes

• How do I add a new binary cache?
• How do I force Nix to re-check whether something exists at a binary cache?
• How to operate between Nix paths and strings?
• How to build reverse dependencies of a package?
• How can I manage dotfiles in $HOME with Nix?
• What’s the recommended process for building custom packages? = "How to build a custom package?" – doesn't have a proper response

Concepts

• What are channels and different branches on github?
• Are there some known impurities in builds?

Start by reviewing the NixOS Wiki FAQ and the listed FAQ sections on nix.dev. Check each question, determine whether it belongs under Troubleshooting, Recipes, or Concepts, and update its answer through a separate PR; done means every listed question has been reviewed and appropriately categorized.