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

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

docs/how-to/migrate-v3-to-v4.md

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+# Migrate from v3 to v4
+
+The main breaking change in v4 is that **short SHA length** is now determined by Git instead of being fixed at 8 characters.
+
+## What changed
+
+Since v4, Git manages the short variables using [`git rev-parse`][git-revparse] behaviour. The length of a short SHA depends on the size of your repository and can change over time as the repository grows.
+
+## How to migrate
+
+To reproduce the previous fixed-length behaviour, set `short-length` to `8`:
+
+```yaml
+steps:
+  - name: Inject enhanced GitHub environment variables
+    uses: rlespinasse/github-slug-action@v5
+    with:
+      short-length: 8 # Same as v3 and before
+```
+
+Other options:
+
+- Set `7` to reproduce small repository behaviour
+- Leave empty to let Git decide (recommended)
+
+> [!WARNING]
+> The minimum length is `4`. The default is the effective value of the [`core.abbrev`][git-core-abbrev] Git configuration variable.
+
+[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