As an alternative of using using a custom document type, you could use Kentico UI strings. It's not as pretty an interface as the forms tab, but you do have the advantage of the resources being quite performant, and multilanguage.
So to set up your 'sitewide' content - create a masterpage and header and footer controls, either aspx or portal and use UI strings for content that won't change very often, but may change from time-to-time.
You can improve usability if you use string key names that explain their location and use - eg: 'homepage.top.header.loginlink.text', 'homepage.top.header.logoUrl', etc