docs(zod-openapi): note route mounting for large RPC apps

[daskodierer]

Summary

Adds a zod-openapi README note about using .route() with small OpenAPIHono apps for RPC apps with many routes.

Refs #1918.

Testing

• bun install --no-save
• bun run prettier --check packages/zod-openapi/README.md
• bun run --filter @hono/zod-openapi lint
• bun run --filter @hono/zod-openapi typecheck
• bun run --filter @hono/zod-openapi test

AI assistance

I used an AI coding assistant for the docs wording and reviewed the diff before committing.

[changeset-bot]

⚠️ No Changeset found

Latest commit: b0d4eb1

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.86%. Comparing base (b504a34) to head (b0d4eb1).
⚠️ Report is 37 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1919   +/-   ##
=======================================
  Coverage   91.86%   91.86%           
=======================================
  Files         115      115           
  Lines        3972     3972           
  Branches     1025     1025           
=======================================
  Hits         3649     3649           
  Misses        287      287           
  Partials       36       36           

Flag	Coverage Δ
zod-openapi	94.90% <ø> (ø)

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