chore(docs): Update latest version (#748)

Update releases page for `5.9.0` release

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Author: jdpleiness

docs/releases.mdx

@@ -12,6 +12,7 @@ Currently supported versions of Sourcegraph:
 
 | **Release**                     | **General Availability Date** | **Supported** | **Release Notes**                                        | **Install**                                          |
 |---------------------------------|-------------------------------|---------------|----------------------------------------------------------|------------------------------------------------------|
+| Sourcegraph 5 Release 9         | October 2024                  | ✅             | [Notes](https://sourcegraph.com/docs/CHANGELOG#v590)     | [Install](https://sourcegraph.com/docs/admin/deploy) |
 | Sourcegraph 5 Release 8 Patch 1 | October 2024                  | ✅             | [Notes](https://sourcegraph.com/docs/CHANGELOG#v581579)  | [Install](https://sourcegraph.com/docs/admin/deploy) |
 | Sourcegraph 5 Release 8         | October 2024                  | ✅             | [Notes](https://sourcegraph.com/docs/CHANGELOG#v580)     | [Install](https://sourcegraph.com/docs/admin/deploy) |
 | Sourcegraph 5 Release 7 Patch 1 | September 2024                | ✅             | [Notes](https://sourcegraph.com/docs/CHANGELOG#v572474)  | [Install](https://sourcegraph.com/docs/admin/deploy) |