docs: Reorganize documentation with new reference and explanation sections (#176)

Co-authored-by: Claude <noreply@anthropic.com>

docs/reference/inputs.md

@@ -0,0 +1,23 @@
+# Inputs Reference
+
+The GitHub Slug Action accepts the following inputs in the `with:` block.
+
+| Input | Required | Default | Description |
+| ----- | -------- | ------- | ----------- |
+| `prefix` | No | `""` | Value prepended to each generated variable name. For example, `prefix: CI_` produces `CI_GITHUB_REPOSITORY_SLUG` instead of `GITHUB_REPOSITORY_SLUG`. |
+| `slug-maxlength` | Yes | `63` | Maximum length of slugified values. Accepts a number or `"nolimit"` to disable truncation. Must not be empty. |
+| `short-length` | No | _(Git default)_ | Length of shortened SHA values. Minimum `4`. When empty, Git determines the length based on your repository size using [`git rev-parse`][git-revparse]. |
+
+## Notes
+
+- When `short-length` is left empty, the action uses the effective value of the [`core.abbrev`][git-core-abbrev] Git configuration variable. This requires your repository to be checked out first (via `actions/checkout`).
+- The `slug-maxlength` default of `63` matches the DNS label length limit, which is also the maximum length for Kubernetes labels.
+
+## See also
+
+- [How to use a prefix](../how-to/use-prefix.md)
+- [How to configure slug max length](../how-to/configure-slug-maxlength.md)
+- [How to configure short length](../how-to/configure-short-length.md)
+
+[git-revparse]: https://git-scm.com/docs/git-rev-parse#Documentation/git-rev-parse.txt---shortlength
+[git-core-abbrev]: https://git-scm.com/docs/git-config#Documentation/git-config.txt-coreabbrev