Add 'Best Practices' section for Addon devs

[luzpaz]

• How to structure addons
  • Avoid name clashes w/ FC core or other addons (Install problem Win10 FredsFactory/FreeCAD_AirPlaneDesign#36 (comment))
  • Recommended linting practices
  • Recommended UI/UX practices
• How to create SVG icons (remove inkscape 'sodipop' metadata)
• How to create experimental branches and orient their testers to take advantage of the Addon Manager feature that allows installing said branches for testing in FC (Proposed code changes for Remote Processing jaheyns/CfdOF#76 (comment))
• How to try to limit 3rd party dependencies when available
• How to translate addons
  • How to add their translations to crowdin

[sliptonic]

Nice! This should be a sub-page under the technical guide