Improve the Hugo documentation to be more accessible and user-friendly

[JerBouma]

With this change, the assumption is made people are mostly interested in the OpenBB Terminal. Therefore, when you open the documentation you are greeted with the following:

What's good to highlight here:

• OpenBB Terminal is automatically always expanded as it is the most relevant menu and it makes sense people see straight away what's inside.
• The documentation you are greeted with is centered around the OpenBB Terminal but does have a few words about the other products.
• Name change from "OpenBB Terminal" to "OpenBB Documentation" at the top. The URL remained unchanged however.
• Renamed each menu to "OpenBB Terminal", "OpenBB Python API" and "OpenBB Bots".

I also made some minor improvements to each menu's texts and adjusted the toc shortcode to also add in a Table of Contents header.