Update Contributing.md participation links

[Contributing.md](https://github.com/internetarchive/openlibrary-bots/blame/7d98fa75e87619990231677db7ceaf1818d1f515/CONTRIBUTING.md#L8) says:
```
## Want to Participate in the Community?
- Ask here to join our Open Library slack: https://github.com/internetarchive/openlibrary/issues/686
- Join us for our Open Library Community Call every Tuesday @ 11:30am PT. We usually discuss weekly goals via our Tuesday Community Call and using Slack.
```

However, the linked issue has been [closed](https://github.com/internetarchive/openlibrary/issues/686#issuecomment-765705467) and recommends going to https://openlibrary.org/volunteer to see how to volunteer.

I'm not sure if the Tuesday call is still happening but if it is then it would be good to mention how to get involved.

Also, I noticed that the link to weekly tasks [here](https://github.com/internetarchive/openlibrary-bots/blame/7d98fa75e87619990231677db7ceaf1818d1f515/CONTRIBUTING.md#L22) is outdated.

Happy to help update these things if people agree that it needs to be changed and how to change it.