[Doc]: In new docs, "Usage guide" entry in the top menu does not link to the "Usage guide"

### Documentation Link

_No response_

### Problem

The top menu "Usage guide" link actually links to the ToC, not to the "Usage guide".

### Suggested improvement

Rename the link or change the link target

### Matplotlib Version

3.5.0.dev2077+g9bfbe9de83

### Matplotlib documentation version

3.5.0.dev2077+g9bfbe9de83