Indicate version in localized docs

[szhorvat]

We have a Spanish tutorial now. Realistically, we can't keep all translated docs up to date ourselves. We need to rely on external contributors.

One way to deal with this would be to indicate the igraph version that the translation is based on. The point is to make it clear to readers, and would-be-translators whether a translation needs an update.

CC @iosonofabio

[iosonofabio]

Sounds like a good idea!

Btw if people do flag it I'm happy to take a stab at updating. My Spanish is ok I think.