docs: add link to site config of Freifunk Dreiländereck (#1425)

docs/user/site.rst

@@ -609,6 +609,7 @@ This is a non-exhaustive list of site-repos from various communities:
 * `site-ffbs <https://github.com/ffbs/site-ffbs>`_ (Braunschweig)
 * `site-ffhb <https://github.com/FreifunkBremen/gluon-site-ffhb>`_ (Bremen)
 * `site-ffda <https://git.darmstadt.ccc.de/ffda/site>`_ (Darmstadt)
+* `site-ff3l <https://github.com/ff3l/site-ff3l>`_ (Dreiländereck)
 * `site-ffeh <https://github.com/freifunk-ehingen/site-ffeh>`_ (Ehingen)
 * `site-fffl <https://github.com/freifunk-flensburg/site-fffl>`_ (Flensburg)
 * `site-ffgoe <https://github.com/freifunk-goettingen/site-ffgoe>`_ (Göttingen)