Expand per-utility documentation with theory sections and worked examples

[YurMil]

Turn the thin utility doc pages (docs/utilities/*.mdx) into self-sufficient reference articles: the underlying theory in plain language (what the standard computes and why), a step-by-step worked example with real numbers, common pitfalls, and a prominent link to run the calculation.

Prioritize by catalog popularity: Blind Flange, Dished End, Shell Rolling, Pipe Cutter, Thread Atlas first.

Why: engineers search for the calculation problem, not the tool name; a page that genuinely explains the calculation serves users better and is discoverable by exactly the audience the tool is for.

Acceptance criteria:

• Top-5 utility doc pages rewritten to the article format
• Each includes a worked example with numbers and units
• Internal links between related tools' articles

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