docs: apply Solid 2 RC follow-ups - #1622

Changed pages include:

• index.html - view
• building-apps/app-structure.html - view
• building-apps/deployment.html - view
• building-apps/environment.html - view
• building-apps/head-and-metadata.html - view
• building-apps/middleware-and-api-routes.html - view
• building-apps/server-functions.html - view
• building-apps/sessions-and-auth.html - view
• building-apps/styling-and-assets.html - view
• concepts/async-reactivity.html - view

There are more changes not included in the list