refactor: structure

[Daanvdplas]

• Have a single welcome (already exists) and getting started page (consolidate current getting started in one page).
• Moved some of the advanced topics in basics into Advanced folder
• Moved storage folder to advanced
• Created development folder for everything regarding development
• Created a reference folder for abi + metadata + macros

The goal is to create a clearer overview.

│
├─ Welcome
│
├─ Getting started
│
├─ 📁 basics/
│ ├─ contract-structure.md (renamed from contract-template.md)
│ ├─ storing-values.md
│ ├─ mutating-values.md
│ ├─ reading-values.md
│ ├─ events.md
│ ├─ errors.md (NEW)
│ ├─ cross-contract-calling.md
│ └─ gas.md
│
├─ 📁 advanced/
│ ├─ 📁 data-structures/
│ ├─ upgradeability.md
│ ├─ xcm.md
│ ├─ precompiles.md
│ ├─ environment.md
│ ├─ selectors.md
│ └─ trait-definitions.md
│
├─ 📁 development/
│ ├─ 📁 testing/
│ ├─ 📁 debugging/
│ ├─ 📁 deployment/
│ └─ 📁 linting/
│
├─ 📁 reference/
│ ├─ 📁 macros-attributes/
│ ├─ 📁 metadata/
│ └─ 📁 abi/
│
├─ 📁 examples/
│ └─ (existing example files)
│
├─ 📁 integrations-and-sdks/
│
├─ 📁 background/
│
└─ 📁 faq/