feat: add usage guide and API file generator

[drnecrotix]

What and why

Adds an in-plugin setup guide and an optional generated API compatibility file for IPS installations that still require a physical PHP endpoint.

Changes

• bump WordPress plugin to 1.1.0
• expand Settings → WP-SSO Bridge into a step-by-step setup guide
• add Generate & Download API file action
• generate wp-sso-api.php without embedding the API secret
• protect file generation with administrator capability and WordPress nonce checks
• document exactly where the generated file must be uploaded: the WordPress root beside wp-config.php and wp-load.php
• show native endpoint and compatibility-file endpoint examples
• update plugin readme.txt and repository README

Security

The generated file contains no credentials. It only loads wp-load.php, verifies the WP-SSO plugin is active, and forwards the request into the maintained plugin endpoint handler.

Expected placement

wp-sso-api.php should be uploaded beside wp-config.php, wp-load.php, wp-admin/, wp-content/, and wp-includes/.