Long document organisation

There is a page on my website that has an already lengthy text in it. I expect to add alot more to it in the near future. It is intended to be a very detailed document. Right now, its all plain text divided into paragraphs. I would like it to be organised so that the visitors can easily jump to a specific section. Also, if the visitor wants to read the whole document from start to end, I don't want to have him scroll from top to bottom, but maybe have some kind of hierarchical tree he can navigate thru. Is there a good tool that can manage this kind of organisation? Or would anyone know of a better way to organise such a lengthy document?