Steal content from DCP-0005

[s-ben]

It looks like @davecgh has created a bunch of new content for the latest DCP,
https://github.com/davecgh/dcps/blob/dcp0005/dcp-0005/dcp-0005.mediawiki
https://github.com/decred/dcps/blob/master/dcp-0005/dcp-0005.mediawiki

Some of it presumably can't be used until the DCP passes (assuming it does). But it looks like there's some background content we could incorporate now? Perhaps the merkle root/trees, SPV, block filter sections?

There could also be some additional reusable content in the DCP archives?
https://docs.decred.org/governance/consensus-rule-voting/consensus-vote-archive/

[davecgh]

Just an FYI that the link to my repo is going to go away once it gets merged to the official DCPs repo, so you'll probably want to update the link here once that happens.