Update release notes for v26.3-v26.3.0-alpha.3

Author: Automated Release Bot

cockroachlabs-preview-main/snippets/releases/v26.3.mdx

@@ -0,0 +1,18 @@
+---
+title: "What's New in v26.3"
+---
+
+import { MarketoEmailForm } from "/snippets/emailform/MarketoEmailForm.jsx";
+import { VersionedLink } from "/snippets/VersionedLink.jsx";
+import V26_3_0_alpha_3 from "/snippets/releases/v26.3/v26.3.0-alpha.3.mdx";
+
+CockroachDB v26.3 release notes.
+
+- For details about release types, naming, and licensing, refer to the <VersionedLink version={version} path="releases">Releases</VersionedLink> page.
+- Be sure to also review the <VersionedLink version={version} path="releases/release-support-policy">Release Support Policy</VersionedLink>.
+
+Get future release notes emailed to you:
+
+<MarketoEmailForm />
+
+<V26_3_0_alpha_3 />