Reformat readme and add table of contents

[yahavi]

• All tests passed. If this feature is not already covered by the tests, I added new tests.
• I used npm run format for formatting the code before submitting the pull request.

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https://github.com/yahavi/setup-jfrog-cli/blob/add-toc/README.md

• Give the README a makeover
• Include a table of contents
• Streamline the OIDC instructions:
  • Keep it super basic - focus on getting it to function for a given repository
  • Drop the nitty-gritty details (like core.getIDToken())
  • Ditch the links to external docs; they clash a bit with this Action
• Enhance the OIDC test a tad.

[eranturgeman]

The updated documentation looks great and the idea to add pictures is very helpful