Add release note for P2A output feature

Author: instagibbs

doc/release-notes-30352.md

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+P2P and network changes
+-----------------------
+
+- Pay To Anchor(P2A) is a new standard witness output type for spending,
+  a newly recognised output template. This allows for key-less anchor
+  outputs, with compact spending conditions for additional efficiencies on
+  top of an equivalent `sh(OP_TRUE)` output, in addition to the txid stability
+  of the spending transaction.
+  N.B. propagation of this output spending on the network will be limited
+  until a sufficient number of nodes on the network adopt this upgrade.