docs: update incentive docs for PIP-55 halving

[b00f]

Summary

Updates the incentive documentation now that PIP-55 (Block Reward Halving) is active.

Changes

• Rewrite the incentive page: "Flat Reward" is now "Block Reward" with the PIP-55 halving schedule table
• Regenerate the Pactus reward charts to show the halving schedule and piecewise-linear reward distribution
• Update the current protocol version to 4 in parameters.md and block.md
• Update reward transaction descriptions in transfer.md and account.md to reflect the PIP-43 split and PIP-55 halving

Verification

• mdl markdown lint: passed
• hugo build: passed
• check_images.py: passed