doc: add example usage for Gitea in tarball fetcher (#11116)

Co-authored-by: Valentin Gagarin <[email protected]>

doc/manual/src/protocols/tarball-fetcher.md

@@ -41,4 +41,30 @@ Link: <https://example.org/hello/442793d9ec0584f6a6e82fa253850c8085bb150a.tar.gz
 For tarball flakes, the value of the `lastModified` flake attribute is
 defined as the timestamp of the newest file inside the tarball.
 
+## Gitea and Forgejo support
+
+This protocol is supported by Gitea since v1.22.1 and by Forgejo since v7.0.4/v8.0.0 and can be used with the following flake URL schema:
+
+```
+https://<domain name>/<owner>/<repo>/archive/<reference or revison>.tar.gz
+```
+
+> **Example**
+>
+>
+> ```nix
+> # flake.nix
+> {
+>    inputs = {
+>      foo.url = "https://gitea.example.org/some-person/some-flake/archive/main.tar.gz";
+>      bar.url = "https://gitea.example.org/some-other-person/other-flake/archive/442793d9ec0584f6a6e82fa253850c8085bb150a.tar.gz";
+>      qux = {
+>        url = "https://forgejo.example.org/another-person/some-non-flake-repo/archive/development.tar.gz";
+>        flake = false;
+>      };
+>    };
+>    outputs = { foo, bar, qux }: { /* ... */ };
+> }
+```
+
 [Nix Archive]: @docroot@/store/file-system-object/content-address.md#serial-nix-archive

src/nix/flake.md

@@ -259,6 +259,8 @@ Currently the `type` attribute can be one of the following:
   `.tgz`, `.tar.gz`, `.tar.xz`, `.tar.bz2` or `.tar.zst`), then the `tarball+`
   can be dropped.
 
+  This can also be used to set the location of gitea/forgejo branches. [See here](@docroot@/protocols/tarball-fetcher.md#gitea-and-forgejo-support)
+
 * `file`: Plain files or directory tarballs, either over http(s) or from the local
   disk.