doc: add release notes

doc/release-notes-31384.md

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+- Node and Mining
+
+---
+
+- PR #31384 fixed an issue where the coinbase transaction weight was being reserved in two different places.
+  The fix ensures that the reservation happens in a single place.
+
+- This PR also introduces a new startup option, `-maxcoinbaseweight` (default: `8,000` weight units).
+  All created block template will reserves the default or specified value for the miner coinbase transaction.
+
+- Bitcoin Core will now **fail to start** if the `-blockmaxweight` or `-maxcoinbaseweight` init parameter exceeds
+  the consensus limit of `4,000,000` weight units.