Fix broken API doc links on site.

[marc0der]

Found some broken links on the following page:
http://gpars.org/webapp/API_Doc.html

[jnorthr]

on our to-do list; When gpars.website server folded, new server had less room to host all prior versions; we may need to restrict ver.s to later gpars vers.

[russel]

Whilst I think we only need to keep the latest bug release version in any series do we need to keep a version of each minor release version, or is it sufficient to keep just the latest in each major release version: so 1.0.x, 1.1.x, 1.2,x, 1.3.x, 2.0.x, etc. or just 1.3.x and 2.0.x. Perhaps we should ensure all the releases are on GitHub and simply point there for all but the most recent in major series.

I never thought we would ever be space restricted, it's 2016 after all. Hence my thinking out loud above.