This adds an encode method into the javascript code so that
users can browser the Godot docs for @globalscope and @GDscript.

The bug was mentioned in:
freeCodeCamp#1853

This adds the encoding on the frontend, rather than trying to override
filename generation in scrapers. It's possible that this will impact
other documentation sources, but I expect those would also need to have
encoded the @ as %40 in their own docs.

This doesn't use JS' encodeURI or encodeURIComponent because encodeURI
doesn't include @, and encodeURIComponent includes / which we don't
want. If there are other URL-reserved characters
that would be useful to encode, those should be easy enough to add to
the short method added here.