docs(builder): clarify Felt division across builder docs

[alimmaster]

Current behavior

The builder docs mention that Felt division uses the multiplicative inverse, but the explanation is fragmented and easy to miss.

As a result, developers can misread Felt division as standard integer division, especially in business-logic scenarios like token amounts, fee calculations, and proportional splits.

New behavior

This change improves Felt division guidance in three places:

• adds a dedicated Division subsection to types.md with side-by-side u64 vs Felt examples
• adds a When to use Felt vs u64 section to patterns.md
• adds a dedicated Felt division pitfall entry to pitfalls.md

Breaking changes

None.

Other info

Documentation-only update.
Closes #232.

[BrianSeong99]

Thanks for this — the core is solid: the math checks out (20/3 → 6148914689804861447, 10/2 → 5), the field-division explanation is accurate, and the u64 vs Felt guidance is genuinely useful. A few changes before merge:

1. Bug — Word has no inner field (pitfalls.md)

In the Solution snippet:

let total_amount: u64 = total.inner[0].as_u64();

Word is accessed via its .a/.b/.c/.d fields (or Index), not .inner — every other Word example in the docs uses field syntax. This won't compile as written. Please change to:

let total_amount: u64 = total.a.as_u64();

2. Trim the over-warning (pitfalls.md)

The entry is a bit heavy: Problem → "especially dangerous" → Why → Solution → plus a :::warning box that largely restates the Solution. Suggest:

• Drop the :::warning Do not use Felt division… callout — Problem/Why/Solution already covers it, and it duplicates the paragraph right above it.
• Soften "especially dangerous" to something like "a common source of bugs." It's well-defined arithmetic, just not what integer-minded code expects.

3. Optional polish

• Keep the divide example consistent: types.md uses felt!(20) / felt!(3), pitfalls.md uses Felt::new(20) / Felt::new(3). Prefer felt!() in both.
• "as_u64() … is zero-cost" (patterns.md) — "inexpensive" is safer; the conversion can involve a canonical reduction.

With #1 fixed and #2 trimmed this is good to merge. Nice addition overall.

[alimmaster]

Addressed the review feedback: fixed the Word access example, trimmed the duplicate warning callout, softened the wording, aligned the Felt division examples on felt!(), and changed zero-cost to inexpensive.