Add bilingual French/English support with Hugo multilingual config

Configure Hugo multilingual mode (fr default, en secondary) with
Hextra's language-switch menu entry, and translate all existing
content pages to English using Hugo's per-file language suffix
convention (.en.md alongside .fr.md).

content/documentation/VXLAN/_index.en.md

@@ -0,0 +1,13 @@
+---
+title: "VXLAN"
+sidebar:
+  open: true
+cascade:
+  type: docs
+---
+
+<!-- markdownlint-disable MD033 MD034-->
+
+{{< cards >}}
+  {{< card link="/en/documentation/vxlan/beginners/vxlan-for-beginners/" title="VXLAN for Beginners" subtitle="Introduction to VXLAN" icon="science" >}}
+{{< /cards >}}