[Docs] Incomplete TODO placeholder in 'Develop Miden in Rust' guide (v0.12)

## Summary

The "Develop Miden in Rust" compiler guide (`versioned_docs/version-0.12/compiler/guides/develop_miden_in_rust.md`) contains an unresolved TODO placeholder in published documentation.

## Problem

The guide states:

> TODO: Add examples of using operations on `Felt` type and available functions (`assert*`, etc.)

This is a development placeholder that was published as-is. The guide covers `Felt` initialization (`felt!` macro and `Felt::new`) but stops short of documenting:

- Arithmetic operations on `Felt` (`+`, `-`, `*`, `/` and their wrapping/modulo behavior)
- Available assertion functions (`assert*`)
- Practical usage patterns combining these operations
- Error handling for values outside the valid field range (0 to 2^64 - 2^32 + 1)

## Impact

Developers following this guide to build Miden programs in Rust reach a gap at the point where they need to actually use `Felt` values in operations. The guide sets up the type but doesn't show how to work with it.

## Suggested Fix

Either:
1. Complete the section with examples of `Felt` arithmetic, assertions, and common patterns
2. Or link to the `miden-stdlib-sys` crate documentation / README where these operations may already be documented

The file to edit is `versioned_docs/version-0.12/compiler/guides/develop_miden_in_rust.md`. Look at the existing content around the `Felt` type and the TODO placeholder. Examine the `miden-stdlib-sys` crate's documentation or source code to understand the available arithmetic operations and assertion functions. Add clear examples of using operations like `+`, `-`, `*`, `/` and functions like `assert*` on `Felt` values. Ensure the examples compile and demonstrate practical usage patterns. Done when the TODO is replaced with complete, helpful documentation.

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