Document signature verification on download page

### Steps to reproduce

### Steps to Reproduce

1. Go to element download page https://element.io/download
2. Click to download a desktop app (eg the `Mac` button)
3. Scroll up & down the page looking for information on how to verify the release
4. ???
5. Get confused and open ticket

### Outcome

#### What did you expect?

I expected the download page to tell me (or link me to the relevant documentation page that does tell me) how to verify the authenticity of the release cryptographically (eg with `gpg`) after the download completes

#### What happened instead?

There's just literally no information on verifying downloads, and it appears that it is not possible to do so.

### Operating system

All

### Application version

All

### How did you install the app?

https://element.io/download

### Homeserver

irrelevant

### Will you send logs?

No