adding more specific verification instructions

[JayOceans]

need to verify fingerprints are authentic by comparing them in multiple places

[asherhopp]

Makes sense to me, the instructions for verifying signatures are somewhat vague as-is.

[katesalazar]

intended or not, this enshrines the for-profit social construct

NAK

[mikeobank]

ACK

[fanquake]

@stickies-v

[maflcko]

Are you still working on this?

[darosior]

Another friendly ping, especially for updating the URL pointing to the builder keys as it's confusing users.

[pinheadmz]

Incorporated into new PR: https://github.com/bitcoin-core/bitcoincore.org/pull/964