[docs] Cross Documentation/Webapp improvement

[rflihxyz]

• [ ] Add informations about custom field data type to Documentation + make video tutorial
• [ ] Reference information inside product when creating custom fields
• [ ] Add explainations about Magic Links Best Practices to docs + make video tutorial
• [ ] Add webapp reference to Magic Links best practices when creating one
• [ ] Add to docs details about custom oauth credentials + reference from product + make video tutorial