doc: release: v18.4.0: add template release notes

Create a template for release notes for the upcoming v18.4.0 firmware
release.

Signed-off-by: Chris Friedt <[email protected]>

Author: cfriedt

doc/release/migration-guide-18.4.0.md

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+# Migration Guide: TT-Zephyr-Platforms v18.4.0
+
+> [!NOTE]
+> This is a working draft for the up-coming v18.4.0 release.
+
+This document lists recommended and required changes for those migrating from the previous v18.3.0 firmware release to the new v18.4.0 firmware release.
+
+[comment]: <> (UL by area, indented as necessary)

doc/release/release-notes-18.4.0.md

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+# TT-Zephyr-Platforms v18.4.0
+
+> [!NOTE]
+> This is a working draft for the up-coming v18.4.0 release.
+
+[comment]: <> (We are pleased to announce the release of TT Zephyr Platforms firmware version 18.4.0 🥳🎉.)
+
+Major enhancements with this release include:
+
+[comment]: <> (H3 Performance Improvements, if applicable)
+[comment]: <> (H3 New and Experimental Features, if applicable)
+[comment]: <> (H3 External Project Collaboration Efforts, if applicable)
+[comment]: <> (H3 Stability Improvements, if applicable)
+
+[comment]: <> (H1 Security vulnerabilities fixed?)
+
+## API Changes
+
+[comment]: <> (H3 Removed APIs, H3 Deprecated APIs, H3 New APIs, if applicable)
+
+[comment]: <> (UL PCIe)
+[comment]: <> (UL DDR)
+[comment]: <> (UL Ethernet)
+[comment]: <> (UL Telemetry)
+[comment]: <> (UL Debug / Developer Features)
+[comment]: <> (UL Drivers)
+[comment]: <> (UL Libraries)
+
+[comment]: <> (H2 New Samples, if applicable)
+
+[comment]: <> (UL PCIe)
+[comment]: <> (UL DDR)
+[comment]: <> (UL Ethernet)
+[comment]: <> (UL Telemetry)
+[comment]: <> (UL Debug / Developer Features)
+[comment]: <> (UL Drivers)
+[comment]: <> (UL Libraries)
+
+[comment]: <> (H2 Other Notable Changes, if applicable)
+
+[comment]: <> (UL PCIe)
+[comment]: <> (UL DDR)
+[comment]: <> (UL Ethernet)
+[comment]: <> (UL Telemetry)
+[comment]: <> (UL Debug / Developer Features)
+[comment]: <> (UL Drivers)
+[comment]: <> (UL Libraries)
+
+[comment]: <> (H2 New Boards, if applicable)
+
+## Migration guide
+
+An overview of required and recommended changes to make when migrating from the previous v18.3.0 release can be found in [v18.4.0 Migration Guide](https://github.com/tenstorrent/tt-zephyr-platforms/tree/main/doc/release/migration-guide-v18.4.0.md).
+
+## Full ChangeLog
+
+The full ChangeLog from the previous v18.3.0 release can be found at the link below.
+
+https://github.com/tenstorrent/tt-zephyr-platforms/compare/v18.3.0...v18.4.0