Documentation style guide update

[TinaKT]

Purpose

To update the existing style guide and include tone of voice guidelines as well.

Changes

The guide is now a document that can be placed on the developer documentation web site under Docs/Help & FAQ/FAQs

It includes information about:

• Documentation types (in accordance with the Divio system)
• Tone of voice and language
• Formatting
• A review checklist to help maintain documentation quality and reduce the risk of common issues

TODO: Update the README of the documentation to reflect the changes - either by including them or to skip them in the latest version or by deleting them from the README.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.

By submitting the contribution I accept the terms and conditions of the
Contributor License Agreement v1.0

• link: https://developers.concordium.com/CLAs/Contributor-License-Agreement-v1.0.pdf

[dragosgd]

LGTM