x/website: link to /dl from project and release history pages

[jmhodges]

It'd be nice to get the download page linked from more places like The Project, Release History, and the release pages.

I always end up on the releases pages first and then don't see a link to find the sha256s and URLs to grab them. It'd be nice to just have link to the downloads page on those

[katiehockman]

/cc @golang/osp-team

As far as I can see, the only link to the downloads page is at the golang.org home page. I agree that this could be in more places, particularly linking from the release pages.