examples: make the vweb_assets.v example more robust (always change work folder to the folder of the executable). Document the vweb.Context.handle_static behaviour.

examples/vweb/vweb_assets/vweb_assets.v

@@ -1,5 +1,6 @@
 module main
 
+import os
 import vweb
 // import vweb.assets
 import time
@@ -16,6 +17,7 @@ fn main() {
 	mut app := &App{}
 	app.serve_static('/favicon.ico', 'favicon.ico')
 	// Automatically make available known static mime types found in given directory.
+	os.chdir(os.dir(os.executable())) ?
 	app.handle_static('assets', true)
 	vweb.run(app, port)
 }