docs: add info about self-hosted github/gitlab instances (#26501)

docs/2.guide/3.going-further/7.layers.md

@@ -100,6 +100,10 @@ export default defineNuxtConfig({
 If you want to extend a private remote source, you need to add the environment variable `GIGET_AUTH=<token>` to provide a token.
 ::
 
+::tip
+If you want to extend a remote source from a self-hosted GitHub or GitLab instance, you need to supply its URL with the `GIGET_GITHUB_URL=<url>` or `GIGET_GITLAB_URL=<url>` environment variable - or directly configure it with [the `auth` option](https://github.com/unjs/c12#extending-config-layer-from-remote-sources) in your `nuxt.config`.
+::
+
 ::note
 When using git remote sources, if a layer has npm dependencies and you wish to install them, you can do so by specifying `install: true` in your layer options.