Add project integrations to nav

Add much of the contents of doc/user/project/integrations/ to the global Nav.

This is a little different to some of the other menus as:

1. I like the Projects > Integrations navigations, but it limits how deep we can go. So I couldn't further split this Project services and Webhooks, with all the project services listed only under Project services. Instead, I've flattened the structure with Project services at the top.

2. Not every available service has a docs page at this point. So I've added what we did have.

3. There's a couple of files in doc/user/project/integrations/ that didn't seem like good fits for the menu. For example:

   • https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/user/project/integrations/jira_cloud_configuration.md
   • https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/user/project/integrations/services_templates.md

4. Not sure what to do with https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/user/project/integrations/prometheus_library/index.md.

At any rate, see what you think. Look ok?