Add docs-devex repo to playbook

[simon-dew]

Part of striping and branching investigation:

• Playbook: this PR
• Devex Capella branch: couchbaselabs/docs-devex#1
• Devex Server branch: couchbaselabs/docs-devex#2
• Server nav: couchbase/docs-server#3032

There would be a similar inclusion in the Couchbase Cloud nav — there is no PR for that change.

[simon-dew]

This PR is not for merge!