Remove arrow and improve button CTA on Onboarding screen

[wojtekn]

Related to https://github.com/Automattic/dotcom-forge/issues/7004

Proposed Changes

I propose to reduce the confusion we observed on the Onboarding screen:

• remove the are that looks like CTA but is not
• change 'Continue' to 'Add site' to make CTA clearer and consistent with the other form

Testing Instructions

1. Remove all sites (or use an appdata-v1.json file with emptied sites)
2. Start the Studio
3. Confirm there is no arrow on left side
4. Confirm CTA is now 'Add site'

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[derekblank]

LGTM. 🚀 I observed that:

• The non-interactable arrow was removed from the Onboarding screen
• The "Continue" button text was updated to "Add site"
• When adding a site, the "Adding site..." text replaced the updated button text as intended
• The "Add site" button text was translated with i18n
• The layout worked as expected for RTL and LTR locales