Update footer link from Gitter to Discord

**Description:**
The footer of the DeepChem website still contains the outdated Gitter community link. Since the community has moved to Discord, this link should be updated to point to the current Discord server.

**Steps to reproduce:**
- Visit the [DeepChem website](https://deepchem.io).
- Scroll down to the footer section.
- Observe that the community link points to Gitter instead of Discord.

**Expected behavior:**
The footer should contain the Discord invite link instead of Gitter.

**Proposed fix:**
Replace the existing Gitter link with the official DeepChem Discord link:
```
https://discord.gg/deXJEfyn5x
```

**Additional context:**
This will help new contributors and users join the correct community channel without confusion.