RFD shortlink should link to RFD site

[benjaminleonard]

Currently the RFD shortlink, e.g. https://{number}.rfd.oxide.computer, takes you to the GitHub file. IMO we're better having the RFD site be the default place that they are viewed.

Also many RFDs use the shortlinks for external references, and this helps avoid jumping from the site to GitHub. We could add another shortlink for GitHub links as a replacement, something like: https://{number}.rfd.oxide.computer/source.