Add custom static links (#234)

docs/configuration/custom-assets.md

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+# Custom assets
+
+To add custom assets to your Opengist instance, you can use the `$opengist-home/custom` directory (where `$opengist-home` is the directory where Opengist stores its data).
+
+### Logo / Favicon
+
+To add a custom logo or favicon, you can add your own image file to the `$opengist-home/custom` directory, then define the relative path in the config.
+
+For example, if you have a logo file `logo.png` in the `$opengist-home/custom` directory, you can set the logo path in the config as follows:
+
+#### YAML
+```yaml
+custom.logo: logo.png
+```
+
+#### Environment variable
+```sh
+export OG_CUSTOM_LOGO=logo.png
+```
+
+Same as the favicon:
+
+#### YAML
+```yaml
+custom.favicon: favicon.png
+```
+
+#### Environment variable
+```sh
+export OG_CUSTOM_FAVICON=favicon.png
+```