Add moneropedia entry for testnet/stagenet

[ghost]

It would be nice to find an explanation of these networks somewhere on the official website instead of having to rely on third party sources. It could be pointed out that the addresses on these networks start with different digits than those on the mainnet.

Also, you should clarify what 'KDF rounds' are and what they have to do with them, because at the moment the word 'KDF' is nowhere to be found on the website except in one of the meeting logs.