Add Delayed Messages API Explainer (#1029)

* Add Delayed Messages Explainer

* Apply feedback and refine Delayed Messages API explainer

- Updated problem description to clarify causes of `postMessage` delays and challenges in identifying root causes.
- Expanded explanation of deserialization timing inconsistencies across browsers.
- Refined summary of problems to emphasize the need for a dedicated API for diagnosing `postMessage` delays.
- Improved description of `PerformanceExecutionContextInfo.name` to clarify optionality for workers and windows/iframes.
- Removed "It adds boilerplate code and maintenance overhead" in manual instrumentation section.
- Added missing reference to the Event Timing API in the references section.
- Update congested example to remove IndexedDB references

* Add Delayed Messages issue template and update README with TOC

* Add 'Related Discussion, Articles, and Browser Issues' and 'Acknowledgements' sections

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Co-authored-by: Joone Hur <joonehur@microsoft.com>

Author: joone

.github/ISSUE_TEMPLATE/delayed-messages..md

@@ -0,0 +1,10 @@
+---
+name: Delayed Messages
+about: new issue
+title: "[Delayed Messages] <TITLE HERE>"
+labels: DelayedMessages
+assignees: joone
+
+---
+
+