Add 'latest' symlink in yt-dl.org/downloads/

[AGSPhoenix]

https://yt-dl.org/downloads/ should have a 'latest' symlink that points to https://yt-dl.org/latest/. Most open source projects that provide a similar list of versions you can download keep a 'latest' link in the same directory, and not having it there is confusing for users that expect that.

I would have assumed the current way was good enough, but someone asked about it in IRC, and it should be easy to fix, so I figured I'd put in an issue about it.

I wrote this submission while rather tired, forgive any wonkiness.

[phihag]

I've added a redirect from /downloads/latest to /downloads/$version. Note that this is different than /latest - the former is just a redirect as commonly seen, whereas the latter provides version-independent filenames.