Rename README to README.md and improve formatting for readability

[swethas274]

This pull request makes the following improvements to the OpenOCD documentation:

1. Renamed the README file from README to README.md for proper Markdown support.
2. Adjusted formatting and alignment in tables (Flash Drivers, Debug Targets, JTAG Adapters) to improve readability and make the documentation more user-friendly.
3. Minor tweaks to headings and spacing for better visual structure and navigation.
4. Ensured Table of Contents links match header anchors for easier access.

These changes are purely documentation-focused and do not affect the OpenOCD codebase or functionality.