Switch links from `<span>` to `<a>`

The links in the developer.html were spans with an onclick
handler that called `prompt` with a URL. This meant to use
the link you had to manaully copy and paste it.

Switching to `<a>` means you can just click the link.
You can still copy the link by right clicking and picking
copy link or long pressing on mobile. The link should still
update to the correct value.