Add multicluster design proposal (#5554)

### Description of the change

Adds to the repository the link to the new design proposal for
multicluster.

### Benefits

Design proposals are properly tracked

### Possible drawbacks

N/A

### Applicable issues

- fixes #5152

Signed-off-by: Rafa Castelblanque <rcastelblanq@vmware.com>

site/content/docs/latest/reference/proposals/README.md

@@ -27,3 +27,4 @@ The proposal process is defined in [proposals.md](./proposals.md).
 | Kubeapps Catalog UI/UX proposal                                        | `antgamdia`    | Evaluate the improvement of the UI/UX when listing the catalog of charts/operators (apps henceforth) from different repositories when they include a large number of elements.              | [Design in GDoc](https://via.vmw.com/2020-kubeapps-ui-proposal)                                                     | Dec 9, 2020   | [#1010](https://github.com/kubeapps/kubeapps/issues/1010)     |
 | Design pluggable support                                               | `gfichtenholt` | Design specification of the pluggable architecture and support for multiple package formats                                                                                                 | [Design in GDoc](https://docs.google.com/document/d/1z03msZRGsRvcRaom-yrvEwIWcEfNy6fSA5Zg28gjYvA/edit?ts=60bb08cb#) | Jun 1, 2021   | [#2944](https://github.com/kubeapps/kubeapps/issues/2944)     |
 | Customizable UI for plugins                                               | `@castelblanque` | Design proposal for the customization of UI only from backend, in order to ease the burden of maintaining UI hardcoded cases per plugin.                     | [Design in GDoc](https://docs.google.com/document/d/1wVYQmYO2TADYOKfabSdLhjnreBjExuB9VxRE8zvvR4o) | Jul 12, 2022   | [#4365](https://github.com/kubeapps/kubeapps/issues/4365)     |
+| New multicluster design                                               | [`@castelblanque`](https://github.com/castelblanque) | A new design for extending multicluster support to all package plugins.                     | [Design in GDoc](https://docs.google.com/document/d/1qYHIa5dd9qwxTdzyNUn7yXacwmYjH7R-KnnWxXkPItE/) | Oct 24, 2022   | [#5152](https://github.com/vmware-tanzu/kubeapps/issues/5152)     |