[Docs] diataxis refactor refactor

[IgorWounds]

1. Why? (1-3 sentences or a bullet point list):

   • The goal is to make the OpenBB Platform easier to navigate and less verbose. User content should go on top of most categories while developer-focused content should be at the bottom.

2. What? (1-3 sentences or a bullet point list):

   • Converts files from markdown to mdx.
   • Renames and splits files.
   • Adds sections to reduce cognitive load.
   • Cuts some verbosity.
   • Changes the ordering and location of files.

3. Impact (1-2 sentences or a bullet point list):

   • Will drastically change the feel and look of the documentation.

4. Testing Done:

   • Ensured it launches without issues and that the pages look and act well.

[deeleeramone]

A few cents to chime in with:

• REST API should go with Getting Started.
• Developer-related stuff under User Guides should go with the Developer stuff.
• Documentation - development should be included in "install from source."
• Enable LLM Mode should be under Getting Started (IMO) - this concept dovetails with "User Settings".

[piiq]

Shall we merge this into the original PR to continue editing and review?