Add custom static links (#234)

docs/configuration/custom-links.md

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+# Custom links
+
+On the footer of your Opengist instance, you can add links to custom static templates or any other website you want to link to.
+This can be useful for legal information, privacy policy, or any other information you want to provide to your users.
+
+To add one or more links, you can add your own file to the `$opengist-home/custom` directory or set a URL, then define the relative path and its name in the config.
+
+For example, if you have a legal information file `legal.html` in the `$opengist-home/custom` directory, and also wish to add a link to a Gitea instance, you can set the link in the config as follows:
+
+#### YAML
+```yaml
+custom.static-links:
+  - name: Legal notices
+    path: legal.html
+  - name: Gitea
+    path: https://gitea.com
+```
+
+#### Environment variable
+```sh
+OG_CUSTOM_STATIC_LINK_0_NAME="Legal Notices" \
+OG_CUSTOM_STATIC_LINK_0_PATH=legal.html \
+OG_CUSTOM_STATIC_LINK_1_NAME=Gitea \
+OG_CUSTOM_STATIC_LINK_1_PATH=https://gitea.com \
+./opengist
+```
+
+## Templating custom HTML pages
+
+In the start and end of the custom HTML files, you can use the syntax to include the header and footer of the Opengist instance:
+
+```html
+{{ template "header" . }}
+
+<!-- my content -->
+
+{{ template "footer" . }}
+```