Update login.mdx

[jsoCloudflare]

Added capabilities of Sign in with Google

Summary

We are adding a Sign in with Google button to the main dashboard login page, and updating documentation to reflect that

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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