Extend Oppiabot's CLA check to include the oppia-web-developer-docs repo

[seanlip]

Is your feature request related to a problem? Please describe.

We would like to have all changes to the main Oppia repos go through our contributor onboarding process, which includes signing the CLA. This includes https://github.com/oppia/oppia-web-developer-docs/ which falls under the Apache license.

Describe the solution you'd like
Oppiabot should extend CLA checking to PRs on the oppia-web-developer-docs repo as well.

Describe alternatives you've considered
N/A

Additional context
N/A