Add conditional to omit the feedback section if necessary

Adds support for omitting the "help and feedback" section from a certain doc tagged on the frontmmater with:

---
feedback: false
---

Please note that none will be affected by this change! It only makes the key available in case we want to use it.

I'll follow this MR by documenting this on https://docs.gitlab.com/ee/development/documentation/structure.html.

cc/ @gl-docsteam FYI