I just looked into this - I think the documentation was not truly unavailable, it's just not available over HTTPS. http://dev.opentripplanner.org
is an AWS S3 bucket served over the web. S3 doesn't allow HTTPS out of the box, only plain HTTP.
I just tried adding HTTPS support via a CloudFront distribution, and this almost works. But unfortunately S3 also doesn't supply HTML directory listings (which we need to allow you to navigate to the directory of documentation you want), so we have always generated those with a script that fetches an XML directory listing from S3 and converts it to HTML. Those XML listings are only available over plain HTTP, making for "mixed content" which is disallowed by current browsers.
Basically, S3 doesn't support HTTPS and directory listings, so I think we'd need to host this documentation somewhere else to get around it.
For the time being we can just always use HTTP URLs instead of HTTPS for this documentation. I am curious, where did you get the URL starting with HTTPS? If we correct that link to one starting with HTTP that should sidestep the problem for the time being.