Document CI pipeline

Describe in details the following steps performed by our CI pipeline and the necessary variables and secrets:

* Main pipeline
  * Install Nix
  * Enable Magic Nix cache
  * Install and configure Cachix
  * `nix flake check`
  * Push to cachix  
* Update Nix flake lock file
  * `APP_ID` and `APP_PRIVATE_KEY` secrets
* Dependabot
  * `CACHIX_AUTH` secret