doc/shortcuts: document permalink y-shortcut

You should never share link externally linking to a branch:

1. line numbers may change
2. the file might get renamed or removed

I did not even find the shortcut documented, even I know it existed forever as I've used it getting used to it from github.

See also:

• https://news.ycombinator.com/item?id=8046710
• https://help.github.com/en/articles/getting-permanent-links-to-files

ps: this deserves a blog post, like github has