Purpose

This is first part of DEV-343: Restructure Concordium Protocol section in docs - Phase 1
This PR covers DEV-344: Restructure Accounts section

Changes

Created two sub-pages for the Account page:

• Account creation
• Account concepts

Moved relevant content from the Account page to the sub-pages

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