docs: fix LaTeX formula rendering and cleanup formatting

[shralex]

• Fixed LaTeX formulas across multiple documentation files by replacing double backslashes with single ones for correct MyST rendering.
• Cleaned up sharding.md and performance_metrics.md formulas and notation.

Tests

Tested by building docs locally.

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.