Replace helm chart with ess-helm instructions

At some point in the near future we'll be removing https://github.com/matrix-org/matrix-hookshot/tree/main/helm and instead suggest users use https://github.com/element-hq/ess-helm to install Hookshot. To be clear, the upstream helm chart will still support standalone Hookshot installation (with an existing homeserver) and will also hopefully do a better job of making it ergonomic to use.

The hope is that with ESS being a well-maintained project this will allow myself and the other Hookshot maintainers to focus on building the software without having to also maintain the Helm chart.

When the helm chart exists for ess-helm, we will deprecate the current helm charts and remove in a future release. Instructions on how to Helm will be included in the hookshot docs.

Start by inspecting the existing helm directory at https://github.com/matrix-org/matrix-hookshot/tree/main/helm and the Hookshot documentation. Check whether the ess-helm chart is available, then update the Helm instructions to point to it while preserving the standalone-installation context. Done means the documentation reflects the intended chart transition without removing the current chart prematurely.

Written by the indexing model from the issue text.