1. Bookmarking is part of the User Model for websites, not the Code Model. In simple English, that means if a user wants to bookmark a page, they'll do it with the built-in browser commands, regardless of a "bookmark link" or not. It would be a useless addition to your page.
2. Since bookmarking is part of the user model, each browser may or may not expose it to the DOM. That means you'll have to write browser-specific code, browser detection code, etc. In other words, you'd be creating yourself a maintenance headache.
3. This is the ASP.NET forum, not the ASP forum.
5. But don't, because if you did I'd simply post to tell you to search [search] "bookmark this page" scripts[/search].