Add warning for outdated docs

[lgoettgens]

See oscar-system/Oscar.jl#3907 for details.
Since Hecke used some non-default design in the past for its docs, and is now using some Vitepress setup, the approach from oscar-system/Oscar.jl#3907 that worked for all other packages mentioned there does not apply here. Nonetheless, it would be great to have something similar for the old Hecke docs as well. In particular, the Vitepress setup does not even mention any version number on the page itself (e.g. https://www.thofma.com/Hecke.jl/v0.31/) such that users can compare the version numbers themselves too realize they use an outdated docs version.