By default WordPress doesn’t provide you the option to add tags in pages, you have this option only for posts. WordPress Plugin Repository has a brand new plugin which automatically adds tags feature to pages and you can display page tags in widget too like you do for posts.
Install and activate this plugin then visit Pages -> Add New page you will notice tags text on right side, following images makes it more clear.
You can add tags while creating or editing your WordPress site pages:
You can manage your page tags like you do in posts:
You can display tag cloud widget containing all your page tags: