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/blog/_index.en.md

@@ -0,0 +1,5 @@
+---
+title: "Blog"
+---
+
+My articles and experience feedback on networking, systems, and DevOps.