Renamed and reorganised smart contract articles

[dragosgd]

Purpose

Review and update Docs Smart Contracts section. The scope of this PR is to give clear names to articles.
On top of this, the 2 Best practices articles were reorganised under 1 section for consistency. An introductory article was written for this.
Also, an obviously unnecessary warning was removed.

Changes

• rename for shorter, more concise, names
• reorganise Best Practices
• write intro article for Best Practices
• remove unnecessary warning

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.
• (If necessary) I have updated the CHANGELOG.

CLA acceptance

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

• I accept the above linked CLA.

[TinaKT]

Really good improvements, shorter headings works much better.

[it09]

Great start!

[it09]

great progress