Delete attrs.html from main graphviz repo

Delete attrs.html from main graphviz repo

This now canonically lives in the graphviz.gitlab.io repo (the docs repo):

https://gitlab.com/graphviz/graphviz.gitlab.io/-/blob/master/doc/info/attrs.html

This gives one central source of truth for the docs, so that people
updating the docs don't have to update 3 locations (doc/info,
doc/infosrc, and the docs repo).

It's perhaps a little controversial to redirect the old docs. The old
docs are still built into some OS's HTML docs packages, but I imagine
the vast majority of users access docs through the graphviz website
after using a search engine to find them.

Fixes https://gitlab.com/graphviz/graphviz.gitlab.io/-/issues/65