Add Alokai customization guide

[mateuszo]

🔗 Linked issue

https://alokai.atlassian.net/browse/DOCS-100

❓ Type of change

• [x] 📖 Documentation (updates to the documentation, readme or JSDoc annotations)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [ ] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)