Add benchmark link to README.rst Fixes #142

[ashwin-r11]

Description

This PR adds a link to the benchmarks on the GitHub Page, making it easier for users to check how the library is performing regarding communication and computation.

Changes made:

• Added a benchmark badge in the header section (alongside existing badges)
• Added a new "Benchmarks" section with a link to the benchmark page at https://openmined.github.io/SyMPC/dev/bench/

Fixes #142

Affected Dependencies

None. This is a documentation-only change to README.rst.

How has this been tested?

• Verified that the benchmark URL (https://openmined.github.io/SyMPC/dev/bench/) is accessible and displays benchmark data correctly
• Verified that the badge renders properly using shields.io format
• Confirmed RST syntax is correct and consistent with existing README formatting

To verify:

1. Check that the benchmark link in the new section works
2. Check that the badge displays and links correctly
3. Preview the README.rst to ensure proper rendering

Checklist

• I have followed the Contribution Guidelines and Code of Conduct
• I have commented my code following the [OpenMined Styleguide](https://github.com/OpenMined/.github/blob/master/STYLEGUIDE. md)
• I have labeled this PR with the relevant Type labels
• My changes are covered by tests