Add vhost examples for popular reverse proxies

We should add some examples to use the stack behind a reverse proxy [here](https://github.com/element-hq/ess-helm/blob/main/README.md#using-an-existing-reverse-proxy). This is particularly important for Apache as that one needs specific config to work properly, namely

```
ProxyPreserveHost on
ProxyPass / http://127.0.0.1:8080/ nocanon
ProxyPassReverse / http://127.0.0.1:8080/
```

Start in README.md at the “Using an existing reverse proxy” section linked by the issue, and review how the Helm deployment exposes the stack. Add examples for popular reverse proxies, including the Apache directives shown in the issue. Done means the README contains usable proxy configurations and clearly explains how to apply them.

Written by the indexing model from the issue text.