Users: Add Home URL to new user registration admin notification email.

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/51792

This PR adds the site's home URL to the admin notification email sent when a new user registers.

Admins who manage multiple WordPress sites often receive new user registration emails with only the site name to identify the source, but two sites can share the same name. The home URL is a unique, unambiguous identifier. This is a UX improvement with no API impact.

Before -

New user registration on your site Example Blog:

Username: johndoe
Email: johndoe@example.com

After -

New user registration on your site Example Blog (https://example.com/):

Username: johndoe
Email: johndoe@example.com

Props to @Ipstenu

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