Migrate Eventing reference to docs-devex

The eventing pages have been migrated from docs-server to docs-devex for release 7.6.

Things that still need to be done:

1. Move the navigation from the docs-server nav to a partial in docs-devex.
2. Change the name of the Eventing service section to "Run a Function on Data Change", as agreed in the Information Architecture plan.
3. Copy the eventing pages and the nav partial into the capella branch, on docs-devex, changing any details necessary for use with Capella.
4. Merge the eventing pages in the capella branch with the existing eventing service branches in the cloud repo, in the same way as the Data service and the Query service.

docs-devex PR link: https://github.com/couchbaselabs/docs-devex/pull/194

couchbase-cloud PR link: https://github.com/couchbasecloud/couchbase-cloud/pull/30632

AV ticket link: https://couchbasecloud.atlassian.net/browse/AV-77417