docs: show core middleware import (#302) + drop $ from copyable shell blocks (#343)

[vishr]

Fixes two long-standing docs issues.

#302 — Missing middleware package import

Core middleware pages showed middleware.CORS(...), middleware.BasicAuth(...), etc. but never indicated which package middleware is, so newcomers couldn't find the import. This adds a prominent import block right after each core middleware page's intro:

import "github.com/labstack/echo/v5/middleware"

• Applied to all 20 core middleware pages (basic-auth, body-dump, body-limit, context-timeout, cors, csrf, decompress, gzip, key-auth, logger, method-override, proxy, rate-limiter, recover, redirect, request-id, rewrite, secure, static, trailing-slash).
• Across all 5 locales (en / zh-cn / ja / es / pt-br). The lead-in sentence is translated per locale; the import code block is byte-identical everywhere, preserving cross-locale code parity.

#343 — Copy button included the $ prompt

session.md showed curl examples with a leading $ prompt inside the code block, so clicking copy grabbed the $ and made pasting awkward. Removed the $ prompt from the command lines (all 5 locales).

Verification

• Production build passes (301 pages).
• Spot-checked translated lead-ins (ja/es/pt-br) and the stripped curl lines; no $ prompts remain anywhere in the docs.

Closes #302
Closes #343

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