Add regtest block generation troubleshooting

[manav8498]

Fixes bitcoin-dot-org/Bitcoin.org#1837.

Summary:

• Adds a troubleshooting section to the regtest block generation example.
• Covers removed or unavailable block generation RPCs, missing loaded wallets, wrong-network addresses, and the bad-fork-prior-to-checkpoint (code 67) error named in the bounty issue.
• Points users toward generatetoaddress, wallet creation/loading, matching -regtest usage, and using a fresh regtest chain/datadir when appropriate.

Verification:

• sphinx-build -M html . _build -n
• git diff --check

Bounty note:

• This PR targets the bounty in Add section with possible error codes and resolution suggestions Bitcoin.org#1837. The payout Bitcoin address should be provided by @manav8498 in a follow-up comment before merge.