Add docs for experimental live collections (#11782)

Co-authored-by: Sarah Rainsberger <[email protected]>
Co-authored-by: Yan <[email protected]>

Author: 3 people

astro.sidebar.ts

@@ -142,6 +142,7 @@ export const sidebar = [
 					'reference/experimental-flags/preserve-scripts-order',
 					'reference/experimental-flags/heading-id-compat',
 					'reference/experimental-flags/csp',
+					'reference/experimental-flags/live-content-collections',
 				],
 			}),
 			'reference/legacy-flags',