Docs feedback: Update Enabling Sentry docs

The docs on enabling Sentry are a bit out of date. This came to our attention via a ticket that is available to GitLab team members with access to Zendesk.

What We Say

Find or generate a Sentry auth token for your Sentry project. Make sure to give the token at least the following scopes: event:read, project:read, and event:write (for resolving events).

What's Not Quite Right

• Pointing to https://docs.sentry.io/api/auth/ to find or generate a Sentry auth token.
• The order in which the scopes are presented could be improved.

What We Should Say Instead

• Find or generate a Sentry auth token. For the SaaS version of Sentry, the auth token can be found or generated by browsing to https://sentry.io/api/. Make sure to give the token at least the following scopes: project:read, event:read, and event:write (for resolving events).

📓 Notes

• We need to account for self-managed Sentry as well as sentry.io.