Added changelog entry

Author: coot

ouroboros-network/changelog.d/20260417_103756_coot_tx_submission_demo.md

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+<!--
+A new scriv changelog fragment.
+
+Uncomment the section that is right (remove the HTML comment wrapper).
+For top level release notes, leave all the headers commented out.
+-->
+
+<!--
+### Breaking
+
+- A bullet item for the Breaking category.
+
+-->
+### Non-Breaking
+
+- Added `ouroboros-network:exe:demo-tx-submission`.
+
+<!--
+### Patch
+
+- A bullet item for the Patch category.
+
+-->