Add frontmatter controlled_document_banner_disabled for optional banner disabling

Problem to solve

The controlled documents banner is helpful for team members to follow the review assignment, and announcement process for more transparency and efficiency.

Some websites such as privacy or terms of service have external facing customers and cannot show the handbook controlled document banner. Therefore, it makes sense to optionally disable the banner through Hugo markdown frontmatter.

Proposal

Introduce controlled_document_banner_disabled as optional second frontmatter attribute for controlled documents. https://handbook.gitlab.com/handbook/editing-handbook/frontmatter/#controlled-documents

Controlled document default

---
title: Handbook Changelog
description: The last 100 Merge Requests to the Handbook
type: docs
controlled_document: true
---

Banner disabled

---
title: Handbook Changelog
description: The last 100 Merge Requests to the Handbook
type: docs
controlled_document: true
controlled_document_banner_disabled: true
---

Banner enabled

---
title: Handbook Changelog
description: The last 100 Merge Requests to the Handbook
type: docs
controlled_document: true
controlled_document_banner_disabled: false
---

Follow-up tasks

1. After approve/merge: Create a new docsy-theme release, and bump the dependencies in the public handbook
2. Add documentation into https://handbook.gitlab.com/handbook/editing-handbook/frontmatter/