Add GDK page for serverless (knative)

Just like we have a separate page for working with Auto DevOps (https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/howto/auto_devops.md) we should have a page for working with serverless and knative.

Probably this page should tell you to follow the steps in https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/howto/auto_devops.md anyway since you will need all the same tunnelling stuff set up for serverless to work.

We should also give an example of a simple project to use for testing purposes that just makes use of kubectl rather than triggermesh like linked in our docs. Since we don't support the triggermesh CLI it makes sense for us to reduce dependencies when doing development and troubleshooting problems.