Update SUMMARY.md

Add the rfq and decimal display guide to the summary, so that it appears as a guide in the Builder's Guide

Author: Liongrass

SUMMARY.md

@@ -137,6 +137,7 @@
   * [Get Started](lightning-network-tools/taproot-assets/get-tapd.md)
   * [First Steps](lightning-network-tools/taproot-assets/first-steps.md)
   * [Taproot Assets Channels](lightning-network-tools/taproot-assets/taproot-assets-channels.md)
+  * [Asset Decimal Display](/docs/taproot-assets/rfq-and-decimal-display.md)
   * [RFQ](lightning-network-tools/taproot-assets/rfq.md)
   * [Collectibles](lightning-network-tools/taproot-assets/collectibles.md)
   * [Universes](lightning-network-tools/taproot-assets/universes.md)