Mark docs as V2 only and link to V3 documentation

[benolayinka]

Let's mark this documentation as V2 specific and link to the V3 docs for The Things Stack.

Here:

And here:

[benolayinka]

@GreenPenguino have you worked on the TTN docs page? Could you help modify the layout to add this note? We want to put a big note at the top of the TTN documentation that it only applies to V2, and add a link to The Things Stack (v3) documentation.

[GreenPenguino]

I have not worked on TTN docs, but willing to try

[benolayinka]

Will be closed by https://github.com/TheThingsNetwork/docs/pull/407