Move .env.install to docs/INSTALL.md as Markdown

docs/CHANGELOG.md

@@ -21,7 +21,7 @@
 - Complete README rewrite with badges, tables, quick start
 - All Dutch comments translated to English
 - XAMPP support removed (security warning added)
-- `.env.example` → `.env.install` with step-by-step guide
+- `.env.example` → `docs/INSTALL.md` with step-by-step guide
 - `.env.standard` → `.env.example.linux` + `.env.example.windows`
 - Nitro V3 update moved from web UI to CLI (Linux-only)
 - Nitro V3 settings moved from database to `.env` file