docs: add metadata placeholder support for Third Party Initiated Login

[dlozlla]

Description

Documents the support for custom domain and organization metadata placeholders in the application-level initiate_login_uri, enabling dynamic Third Party Initiated Login (TPIL) URLs based on the custom domain or organization context of the request.

Changes across three files:

• configure-default-login-routes.mdx — New "Dynamic login URIs with metadata placeholders" section covering supported placeholders ({custom_domain.metadata.public_KEY} and {organization.metadata.public_KEY}), fallback behavior when placeholders cannot be resolved, and an explicit note that placeholders are not supported on the tenant-level default_redirection_uri.
• wildcards-for-subdomains.mdx — Added Application Login URI (initiate_login_uri) to the list of supported fields for custom domain URL placeholders. Also fixed a typo ("Logour" to "Logout").
• multiple-custom-domains.mdx — Added "Application URL placeholders" subsection linking to the wildcards page for details on using custom domain metadata in application URLs.

References

• https://oktainc.atlassian.net/wiki/spaces/IAMPS/pages/671586345
• https://auth0team.atlassian.net/browse/IPS-5572

Testing

Run mint dev from the main/ directory and verify the following pages render correctly:

• /docs/authenticate/login/auth0-universal-login/configure-default-login-routes
• /docs/get-started/applications/wildcards-for-subdomains
• /docs/customize/custom-domains/multiple-custom-domains

Checklist

• I've read and followed https://github.com/auth0/docs-v2/blob/main/CONTRIBUTING.md.
• I've tested the site build for this change locally.
• I've made appropriate docs updates for any code or config changes.
• I've coordinated with the Product Docs and/or Docs Management team about non-trivial changes.