Invalid address for Helm repo

### Description

- **What** is the problem: cannot add https://element-hq.github.io/dendrite as a valid Helm repo
- **Who** is affected: k8s users
- **How** is this bug manifesting: HTTP 404 on https://element-hq.github.io/dendrite/index.yaml
- **When** did this first appear: since the move to the new repo?

### Steps to reproduce

- `helm repo add dendrite https://element-hq.github.io/dendrite/`
- The following error message will appear:
```
Error: looks like "https://element-hq.github.io/dendrite" is not a valid chart repository or cannot be reached: failed to fetch https://element-hq.github.io/dendrite/index.yaml : 404 Not Found
```

Compared to https://matrix-org.github.io/dendrite/index.yaml, the Helm chart index file is not stored in the docs website.

Start by comparing the working matrix-org.github.io/dendrite/index.yaml with the element-hq.github.io/dendrite documentation-site and Helm chart publishing entry points. Done means the index.yaml is available at the new URL and `helm repo add dendrite https://element-hq.github.io/dendrite/` succeeds.

Written by the indexing model from the issue text.