GH-16182: Updating user guide page Welcome to reflect makersaurus guidelines

[hannah-tillman]

#16182

This PR is a large overhaul over the Welcome page structure. A while ago I was also tasked with making the Welcome page less overwhelming which is why this is a more in-depth update than any of the other pages will be.

• Kept basic framework explanation and requirements all on initial welcome page
• Gave all user-types their own pages (structured similar to the algorithms section)
  • Some pages (like Flow's and API's) are very small pages because of this
• Broke up bulkier sections
• Standardized headers
• Improved link text and alt text
• General fixes/adherence to style guide

Any feedback is super welcome! This is a bigger change than normal, but breaks down the information into more digestible chunks. Let me know your thoughts :)

[hannah-tillman]

@wendycwong I tested all the links again and they're still working on my end. I don't know why they aren't working for you :( Is anyone else having this issue?