docs: add self-hosted runner setup (closes #937)

.github/readme/partials/documentation/selfhosted.md

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+# 🏠 Run metrics on self-hosted runners
+
+## 0️ Setup a self-hosted runner
+
+Learn more about hosting runners on [GitHub documentation](https://docs.github.com/en/actions/hosting-your-own-runners).
+
+> ⚠️ To run *metrics* on a self-hosted runner, the following dependencies needs to be installed on runner:
+> - [docker](https://www.docker.com)
+> - [jq](https://github.com/stedolan/jq)
+
+Go to repository settings, and select `Runners` under the `Actions` side tab.
+
+![Add a self-hosted runner](/.github/readme/imgs/setup_selfhosted_create.light.png#gh-light-mode-only)
+![Add a self-hosted runner](/.github/readme/imgs/setup_selfhosted_create.dark.png#gh-dark-mode-only)
+
+> ⚠️ Working user **must be able** to run docker. If *metrics* is run with an unprivileged user, ensure it can open `/var/run/docker.sock`
+>
+> Use the following workaround when receiving the following error: `dial unix /var/run/docker.sock: connect: permission denied`
+> ```bash
+> usermod -a -G docker $USER
+> chown root:docker /var/run/docker.sock
+> ```
+
+## 1️ Update workflows to use self-hosted runners
+
+To run *metrics* action on a self-hosted runner, uses `runs-on: self-hosted`.
+
+*Example: render metrics for `github` organization*
+```yaml
+runs-on: self-hosted
+steps:
+  - uses: lowlighter/metrics@latest
+    with:
+      token: ${{ secrets.METRICS_TOKEN }}
+```
+
+> 💡 To easily debug workflow errors, use [`debug: yes`](https://github.com/lowlighter/metrics/tree/master/source/plugins/core#debug) option