Add docs for R2 SQL

[laplab]

Proposed changes

• https://developers.cloudflare.com/workers/wrangler/system-environment-variables/ to include new variable name
• https://developers.cloudflare.com/r2/ to include new page for R2 SQL open beta
• https://developers.cloudflare.com/r2/sql/ - url of the new page, contains overview of the product, it's pricing, performance considerations and best practices

Subject Matter

R2 SQL

Content Location

New page

Additional information

No response

[laplab]

Once the documentation is published, we should also add a link to it in the error we give in wrangler r2 sql query when the token env variable isn't set.