Further website improvments

Requested by Sid after #1302 (comment 28483806)

1. Move https://pages.gitlab.io/ under /features
2. 'Learn more' is not descriptive, as a user I want to know when I'm leaving the website to go to the docs, it should say 'Documentation' or 'Feature details' depending on where the link goes.
3. Video on https://about.gitlab.com/features/ should be updated to GitLab 9.0 release one.
4. Rename CI to "CI/CD Pipelines" (requested by Mark P. #1302 (comment 28701785))