Add a notice that current version of docs is for v3.

[senadir]

I spend a good 20mn debugging why useQueryClient is not being exported from react-query before discovering that I installed react-query a week ago in version 2.6 and the docs are currently showing v3 examples, a small notice or dropdown (that you're viewing v3 docs, see v2 docs) would be helpful to remind people to check versions.

Thank you for the hard work, this is not a complain, but a suggestion.

[mdanics]

For anyone who came here from Google, what you're looking for is useQueryCache

[tannerlinsley]

We're working on a doc switcher :)