docs: add massCode vs Postman comparison page

[antonreshetov]

Adds /compare/postman — the seed page for the HTTP/API territory (Track B).

Honest framing: Postman is a full cloud-first API platform; massCode's HTTP space is a lightweight, local-first API client inside a workspace. No outdated "Postman forces login" angle — verified that since May 2023 Postman offers a no-sign-in lightweight client (Scratch Pad deprecated). Differentiators are local-first plain files, no account, free/open-source, all-in-one, and the 2026 free-tier tightening.

• All Postman facts verified from Postman docs/pricing at write time (lightweight client + offline behavior; 2026 free plan = 1 user, capped collection runs; paid from ~$14/user/mo).
• Strong migration story grounded in the real importer: massCode imports Postman Collection v2.1 + Environment JSON (per imports.md).
• massCode HTTP capabilities grounded in the docs (requests, environments, copy as cURL); honest trade-offs (no mocks/monitors/runner/scripts, HTTP-only, plain-text storage warning).
• Registered in compare sidebar and index.

Note: the first-person flagship post "Why I built my own HTTP client" is intentionally NOT included — it needs the author's real motivations/voice rather than invented narrative.