Include API name in page title

[mattt]

All generated pages have the same title. In the case of the libgit2 documentation, it's "libgit2 API". It would be helpful for the <title> element to include the name of the API documented on that page, as that would allow tabs and bookmarks for different APIs to be distinguished from the browser. For example, the page for git_cherrypick could have the title "libgit2 API - git_cherrypick".