chore: add ExpiresInMinutes as an email template variable

[TamerShlash]

Related Issue or Design Document

• PR to introduce the ExpiresInMinutes field feat: passwordless SMS and expiry notice in code / link templates kratos#4104

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact [email protected]) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation within the code base (if appropriate).

Further comments

As the title states, this is a simple suggestion to add the template variable to the docs.

[vinckr]

thanks for adding that 🐝
lgtm!