Removed checksum ledger & updated README

[coolamit]

Made significant improvements to the README.md, clarifying features, configuration and differences from the upstream OCR action. The changes add new sections, improve documentation on concurrency and permissions and provide more detailed explanations of comment budgeting, file exclusion and trigger modes.

Documentation improvements:

• Expanded the table and explanations comparing Themis to the upstream OCR GitHub Action, clarifying deduplication, merge gating, comment budgeting, file exclusion, supply chain, and manual trigger differences.
• Added and reorganized the table of contents for easier navigation, and introduced new sections such as .themisignore details, trigger modes, and provider recipes.

Workflow and configuration guidance:

• Added concurrency and timeout-minutes settings to the workflow example, and clarified required permissions (notably actions: read) for correct operation and label handling.
• Improved explanations of the label triggered review flow, including how duplicate reviews are avoided and how Dependabot PRs are handled.

Feature behavior clarifications:

• Clarified the behavior of the severity gate, comment budgeting (including critical finding handling and overflow summaries), and how suggestions are posted only when the code matches.
• Updated descriptions of the exclude input and .themisignore file, including failure conditions, case sensitivity, and the interaction between the two exclusion mechanisms.

These changes make the documentation more comprehensive, accurate, and user-friendly, helping users to configure and understand Themis more effectively.