Update providing own contact details and fix broken links

[emmagifford]

Description

I have:

• fixed 3 broken links in the 'Before you confirm your setup' section in the 'Confirm your use case' accordion, on /get-started/onboard-with-nhs-notify
• updated 'Providing your own contact details for recipients' in /using-nhs-notify/tell-us-who-you-want-to-message

Context

James spotted the 3 broken links and requested this fix.

Kas, our IG expert, asked for the change to 'Providing your own contact details for recipients' following a request from legal - see CCM-16490CCM-16490 for more context.

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

---

Sensitive Information Declaration

To ensure the utmost confidentiality and protect your and others privacy, we kindly ask you to NOT including PII (Personal Identifiable Information) / PID (Personal Identifiable Data) or any other sensitive data in this PR (Pull Request) and the codebase changes. We will remove any PR that do contain any sensitive information. We really appreciate your cooperation in this matter.

• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.