docs: update README with new config format and features from recent PRs

[Copilot]

Summary

Updates README.md to reflect three recently merged PRs that changed the config format significantly. The audience is server admins installing and configuring the addon.

Changes

• Options section moved to the top of the Configuration section so admins see the master switches (enable/disable, tree types) before diving into block replacements
• Block replacement format updated from the old single-string format (logs: gravel) to the new weighted map format (logs: { gravel: 100 }) matching the actual config.yml
• Weighted block mixing documented with rules for total weights = 100, > 100, and < 100
• Per-tree-type overrides section added — explains how to give individual species different block replacements
• Per-gamemode overrides section added — explains how to configure different blocks per BentoBox gamemode (BSkyBlock, CaveBlock, etc.)
• Priority order summary added showing how per-tree → per-gamemode → global default resolution works

Related PRs

• Add per-tree-type material overrides for Nether and End dimensions #41 — per-tree-type overrides
• feat: per-gamemode block configuration for Nether/End trees #42 — per-gamemode overrides
• Add weighted multi-material mixing for log/leaf replacement #43 — weighted multi-material mixing