docs: unslop pass over Learn pages and tutorials - #1685

Changed pages include:

• index.html - view
• tutorial/index.html - view
• reference/index.html - view
• glossary/index.html - view
• 404.html - view
• building-apps/head-and-metadata/index.html - view
• building-apps/middleware-and-api-routes/index.html - view
• building-apps/server-functions/index.html - view
• building-apps/sessions-and-auth/index.html - view
• building-apps/styling-and-assets/index.html - view

There are more changes not included in the list