Feedback on the chapter on dependent_type_theory.md

[lovettchris]

• Fix how some anchor links work ( no need to use HTML <a href=... ). Notice that every heading generated by mdbook automatically has an anchor tag whose name is that heading in lower case with space replaced by underscore. You can also find that link by simply clicking the heading and the link appears in the browser address bar,
• Fix pair projection syntax.
• Add something about dot notation.
• Some wording improvements to make some things more explicit.
• Delete 'test' file which is no longer needed by mdbook.