Various Tutorials section improvements, other minor polish

[benitav]

• Consistent “self-host” spelling across docs (with the hyphen)
• Consistent "demo app" vs "example app" and "to-do list" vs "todo list" spelling
• Add high-level overview page for Tutorials
• Also link to other relevant content from the Tutorials overview page
• Remove the “backend - coming soon” tutorial group and more globally explain that more content is coming soon
• Cross reference to Tutorials throughout docs where relevant
• Add accordions to Demo apps page to prevent excessive scrolling
• Cross reference between tutorials, use case examples and demo apps (better combining them can be tackled in a future PR)

[HeinrichvonStein]

Thanks for these changes! The new overview page was an excellent idea! My only nit is if we could remove the 'Tutorial' section on the left but not sure if mintlify actually supports doing that.