Add keys to site-pages

[arunshenoy99]

Proposed changes

This adds a new key to the site-pages array, which will be used across languages to identify and report events on the site pages step.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc
• Linting and tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

This goes with: newfold-labs/wp-module-onboarding#575