docs: Added release notes and associated media for 0.38 (apache#12022)

* Added release notes and associated media for 0.38

* Added ASF license header to release notes

* refactored the release notes directory structure. Re-added media to the 0.38 release notes.

* fixed broken image and hyperlink

* fixed broken image and hyperlink pt 2

* Swapped out media embed tags

Swapped out github style markdown embed tags with html style ones for more control.

* Fixing small inconsistencies

Fixed a couple small inconsistencies in the feature tags

* Resize media, update embed style

Seems like GitHub does not any but the most basic css in the embed tag.

* Spelling nit

* Fixed some formatting nits

Added a box around the images to clearly distinguish them from the whitespace. Images are now embedded on the same indentation tier as their associated bullet point.

* Rebase to hopefully fix cypres problems

* May the cypress gods be pleased with this commit.

README.md

@@ -41,6 +41,7 @@ A modern, enterprise-ready business intelligence web application.
 [**Why Superset**](#why-superset) |
 [**Supported Databases**](#supported-databases) |
 [**Installation and Configuration**](#installation-and-configuration) |
+[**Release Notes**](RELEASING/release-notes-0-38/README.md) |
 [**Get Help**](#get-help) |
 [**Contributor Guide**](#contributor-guide) |
 [**Resources**](#resources) |