Add support for custom link headers in examples
Fix example link headers for Events API

By default, the docs show a "next" and "last" link in the link header.
However, the Events API only provides "first", "prev", and "next" links
in the link header. Therefore, when you get the first page, the link
header only includes a "next" link. Update the example response to
reflect that behavior.
Add link header to /repositories example
Refactor #link_header
Add link header to /users example
Add link header to commit listing example
Improve link header examples