[Internal]: Document basic authentication support for xpack.actions.proxyUrl

### Description

Kibana is adding support for basic authentication on the proxy configured via `xpack.actions.proxyUrl`. Previously, the docs explicitly noted that basic authentication was not supported for this setting. This change removes that limitation.

Documentation should be updated to:
- Remove or update the note stating that basic auth is not supported for `xpack.actions.proxyUrl`
- Document the new configuration options for specifying proxy credentials (username/password)
- Provide an example of a full proxy URL with basic auth credentials

### Resources

This feature was requested in https://github.com/elastic/enhancements/issues/20568.

Related enhancement: https://github.com/elastic/enhancements/issues/18277

Current docs page that needs updating: https://www.elastic.co/guide/en/kibana/current/alert-action-settings-kb.html

### Which documentation set does this change impact?

Elastic On-Prem and Cloud (all)

### Feature differences

The feature is identical in all deployment methods.

### What release is this request related to?

N/A

### Serverless release

Unknown — check with the ResponseOps team.

### Collaboration model

The documentation team

### Point of contact.

**Main contact:** @

**Stakeholders:** Team:ResponseOps