Update and clean up readme

[gschueler]

This pull request updates the README.md to better reflect the current capabilities and usage of the Rundeck HTTP Workflow Step Plugin. The documentation now clarifies that the plugin is bundled with Rundeck, expands on supported authentication methods, and provides improved usage instructions.

Documentation improvements:

• Clarified that the plugin is included with Rundeck and maintained as part of the main rundeck/rundeck project, removing references to external installation.
• Expanded the list of supported authentication methods to include Basic, Bearer Token, and OAuth 2.0 (Client Credentials Grant), and clarified supported HTTP methods.
• Added a usage section with instructions and a link to the official documentation for configuration details.

Other updates:

• Updated caveats to clarify OAuth 2.0 support and credential sharing.
• Removed outdated badges and the "Todo" section.

[Copilot]

Pull request overview

Updates the project README to modernize and clarify how the Rundeck HTTP step plugin is used and what capabilities it provides.

Changes:

• Refreshes the introduction to mention both workflow and node step usage and updates Rundeck links.
• Expands the feature list (including Bearer token auth) and adds a concise Usage section with a link to Rundeck documentation.
• Removes old CI/status badges and trims outdated sections.

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