docs: add support for WEBSITE_THEMe env variable

Author: Sébastien Chopin

docs/.env.example

@@ -0,0 +1,2 @@
+# Local version of the website theme
+WEBSITE_THEME=/my/path/to/website-theme

docs/nuxt.config.ts

@@ -1,5 +1,7 @@
+import{join}from'pathe'
+
 exportdefaultdefineNuxtConfig({
-extends: '@nuxt-themes/website',
+extends: process.env.WEBSITE_THEME ? join(process.env.WEBSITE_THEME,'theme') : '@nuxt-themes/website',
 nitro: {
 prerender: {
 routes: ['/','/404.html','/guide/directory-structure/app.config']