Display descriptive titles on guide pages

Created by: ProkhorZ

The Developer Guides and many of the User Guides do not show at the top of the page which question they're answering. They start either without any heading, with a subheader like Introduction or with a summary of the question, like Prove payments. It's confusing that these pages don't clearly show what they're about, especially when new users are given a direct link to one of these pages to answer a question.

There is a title element which is used for the <TITLE> tag, but this is hardly visible in a modern tabbed browser or a mobile browser. @rehrar Could you use the same text in a visible <H1> or <H2> tag at the top of each page?

Such a change is conveniently beyond my capabilities, but I guess I could clean up any duplicate titles that resulted from this.