Update Epoch 2.5 and Epoch 3.0 example node/signer configs and docs

[jferrant]

To run mock signing/mock mining 2.0 in Epoch 2.5, a mining_key MUST be set. e.g. [miner] #pre_nakamoto_mock_signing = true min_tx_fee = 1 first_attempt_time_ms = 5 #subsequent_attempt_time_ms = 30_000 wait_for_block_download = false mining_key = "[[node.key]]" wait_on_signers_ms = 20_000 activated_vrf_key_path = "/stacks-blockchain/data/saved_vrf_key.json"

We probably need to similarly update Epoch 3.0 to include a mining key. It is required for post nakamoto to run at all.

There probably are other config options I am forgetting, but mining_key is definitely an important one to mention.

[wileyj]

i can add some of this to my open PR: https://github.com/stacks-network/stacks-core/pull/4995

but we'll still want to loop in @kenrogers so the website docs are also updated

[blockstack-devops]

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