Use absolute link to the manual

A ton of people (see list below) click this link on GitHub, and are lead to a
404 page. You could link to the markdown page, as is suggested at #253, but
that won't work at the cx4a page, and anyway, that page is better rendered
than the GitHub version.

Closes #253, closes #263, closes #262, closes #236, closes #237.
Use the correct link to the manual
Merge pull request #264 from asmeurer/manual

Use absolute link to the manual