Update README for beta

[libutcher]

This MR updates the README for the beta. It includes the following changes:

• Some improved styling, like a centered header and beta callout
• Links to documentation
• Edits to reflect edits made for beta docs, like changing "read access" to "access", updating the warning callout text, etc.
• Separates authentication & limitation info into their own sections
• Updates get started steps to reflect beta docs changes (for example, removed any steps that are no longer needed for the beta)
• Rephrases token provision section to reflect beta docs changes, explaining using the OP_SERVICE_ACCOUNT_TOKEN environment variable as an option
• Adds PowerShell examples

Question for reviewer: Does the "Use the SDK in your project" code example need to be updated?