Links to doc are broken in v14.x

[vhenzl]

For example error like this points to a page that no longer exists:

Schema must contain unique named types but contains multiple types named "ID". Make sure that type loader returns the same instance as defined in User.id (see http://webonyx.github.io/graphql-php/type-system/#type-registry).

Because v14 is the current stable version, changes from #818 (and consequent commits) should be backported.

Alternatively, or even better in addition to that, the doc website should redirect old URLs to the new ones, so that links keep working for everyone even without updating graphql-php, and also for all links in the wild. (Not sure though how or if that can be done in mkdoc.)