Mark deprecations in HTML

For example NXroot has

```
<attribute name="NeXus_version" deprecated="NAPI is frozen.">
```

The HTML rendering does not show this

<img width="756" height="126" alt="Image" src="https://github.com/user-attachments/assets/0be7092e-fd83-4bf6-80b7-e5afb07cf8fc" />