Add a 'homepage' field to the website model

In the digital registry, websites are defined by their `domain`. For most sites, https://domain loads the homepage but there are some exceptions where the `/` path is not defined. Add an optional `homepage` field to support such sites.

**Example:**
Domain: pegasys-pub.gsa.gov
Homepage: https://pegasys-pub.gsa.gov/momex

No file or test is named. Start by locating the website model and its schema or validation entry point, then inspect nearby tests for website fields. Done means an optional homepage value supports URLs such as the provided example while existing domain-based websites continue to work.

Written by the indexing model from the issue text.