Document regtest block generation errors

[Randy-sin]

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

This adds a troubleshooting table to the Regtest Mode section for block-generation failures.

Covered cases:

• bad-fork-prior-to-checkpoint / code 67, including checking that both bitcoind and bitcoin-cli are using -regtest and refreshing stale regtest chain state when needed.
• Method not found for the removed generate RPC, pointing users to generatetoaddress.
• No wallet is loaded when getnewaddress is used before creating/loading a regtest wallet.
• Error: Invalid address when generatetoaddress is given a missing, mistyped, or wrong-chain address.

Validation:

• git diff --check
• make html succeeded locally

Note: I did not include a payout address in the PR body to avoid publishing payment information.