docs: add remaining docs back with some additions/improvements

docs/cicd_component.md

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+# CI/CD Component
+
+The CI/CD Component has some inputs which configure defaults for Nix GitLab CI.
+
+## `version`
+
+- Type: `string`
+
+Which version of the Nix CI image to use. Using a tag/version is recommended.
+Will not do anything if a custom image is specified using `NIX_CI_IMAGE`.
+
+## `cache_strategy`
+
+- Type: `string`
+- Default: `"auto"`
+- Options: `auto` | `none` | `runner` | `cachix` | `attic`
+
+Sets the default caching strategy.
+
+- `auto`: dynamically selects the best strategy for every job based on env variables
+- `none`: disables caching
+- `runner`, `cachix` & `attic`: forces every job to use this strategy
+
+Can be overridden by `NIX_CI_CACHE_STRATEGY`, see [Environment Variables](./environment_variables.md).
+
+## `cache_files`
+
+- Type: `array` (of strings)
+- Default: `["flake.nix", "flake.lock"]`
+
+Files to use as the cache key for the generated pipeline yaml.
+If you use a file like `ci.nix` to define CI, add that here for example.
+This makes sure that changes to your Nix CI configuration will invalidate the cache,
+otherwise an old pipeline yaml might be used.
+
+!!! warning
+
+    The value of this is used in `cache:key:files`, which currently only supports
+    a max of 2 entries. So use something like `["flake.*", "ci.nix"]` to match
+    `flake.lock`, `flake.nix` and `ci.nix`.
+    See [gitlab-org/gitlab#301161](https://gitlab.com/gitlab-org/gitlab/-/issues/301161)