Add warning to old versions of docs

[szhorvat]

A lot of google hits still go to the old versions of Python and R doc pages, e.g.:

https://igraph.org/r/doc/layout_with_lgl.html

https://igraph.org/python/api/0.9.11/igraph.Graph.html

If feasible and easy, it'd be useful to indicate that these docs are outdated with a banner like the one currently used in the new Python docs: