docs: add remaining docs back with some additions/improvements

docs/caching.md

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+# Caching
+
+Nix GitLab CI supports several caching mechanisms to speed up your pipelines.
+
+## GitLab Runner Cache
+
+The runner cache strategy copies the new store paths into a directory `.nix-cache`,
+which is then saved in the regular GitLab cache (technically runner cache).
+It's also configured as a substituter automatically.
+
+To enable, set the cache strategy to `runner`.
+
+Configure it using these environment variables:
+
+- `RUNNER_CACHE`: path to the runner cache (default `.nix-cache`)
+
+!!! warning
+
+    This is very inefficient and should probably only be used for very very small
+    dependency counts. Otherwise it takes an eternity to save to cache.
+
+## Cachix
+
+Cachix is a hosted binary cache service that can significantly speed up Nix
+builds by sharing build results.
+
+To enable, set the cache strategy to `attic`.
+
+Configure it using these environment variables:
+
+- `CACHIX_CACHE`: name of the cache to use
+- (`CACHIX_AUTH_TOKEN`): cachix client itself uses this for authentication
+
+!!! warning
+
+    Cachix has not been tested. Feedback is appreciated :)
+
+## Attic (Self-Hosted Cache)
+
+Attic is a self-hosted, deduplicating binary cache. It's a great option if you
+want more control over your caching infrastructure and to have the cache closer
+to your runners.
+
+To enable, set the cache strategy to `attic`.
+
+Configure it using these environment variables:
+
+- `ATTIC_SERVER`: URL of the server
+- `ATTIC_CACHE`: name of the cache to use
+- `ATTIC_TOKEN`: auth token from the attic server