Update How to Stake for vspd

[xaur]

How to Stake still talks about registering with VSP and redeem scripts. This info needs to stay for legacy VSP users, but new vspd info needs to be added and put forward.

It might not hurt to mention mixed staking (without any wallet specifics, which should live on dcrwallet/Decrediton pages).

[xaur]

@jholdstock I see that #1171 made some vspd-related updates but it did not touch the How to Stake page reported in this issue. Same for its sibling Redeem Script page. I think they need a mention that they describe the legacy model (but they don't need to be removed for now, imo).