Docs: fix typos and add NEXT_SITE_URL to env variables

[mosesintech]

This PR will

• Fix the typo in the WordPressTemplate templates example, from archive: { post: ... } } to archive: { posts: ... } }.
• add NEXT_SITE_URL to Environment Variables docs.

Issue:

Without the NEXT_SITE_URL, if devs have followed the docs for adding @nextwp/core to their project they will receive the following error:

 ⨯ Internal error: Error: No NEXT_SITE_URL provided. Please set NEXT_SITE_URL env

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated (UTC)
next-wordpress-starter	✅ Ready (Inspect)	Visit Preview	Feb 13, 2024 0:41am
nextwp-docs	✅ Ready (Inspect)	Visit Preview	Feb 13, 2024 0:41am

[CalebBarnes]

Amazing, thank you!

[CalebBarnes]

Also, right now we are using NEXT_SITE_URL in some functions like the sitemap urls, menu items, archive urls, etc, but I plan on removing the need for this. For now we should definitely have that documented though!

Ideally we can just detect the frontend url from Netlify or Vercel, and if it is deployed elsewhere then this one could override it.

Will also consider #26 before changing anything related to the urls.