docs: add staging captcha workaround for Desktop

[autrin]

First time contributor checklist:

• I have read the README and Contributor Guidelines
• I have signed the Contributor Licence Agreement

Contributor checklist:

• My contribution is not related to translations.
• My commits are in nice logical chunks with good commit messages
• My changes are rebased on the latest main branch
• A pnpm run ready run passes successfully (more about tests here)
• My changes are ready to be shipped to users

Description

Adds a short hCaptcha workaround note to CONTRIBUTING.md → The staging environment.

Fixes #7304

What’s changed

• Documents the current limitation where development/Electron builds do not capture the
  signalcaptcha://… callback automatically.

• Provides the exact command-line workaround:

  pnpm start -- "signalcaptcha://signal-hcaptcha-short-XXXXXXXX"
  

so new contributors can unblock registration or relinking on staging immediately.

Value to users

Without this note, first-time builders are stuck on the phone number page and often assume the project is broken.
The added two-sentence tip saves at least 10-15 minutes of head-scratching and reduces duplicate support questions in Issues.

Test approach

• Manual-docs test – rendered the updated markdown in preview mode; verified the new call-out appears after the Relink → Standalone step.
• Ran pnpm run ready → everythin pass (no runtime code touched).
• No new automated tests were necessary because this is a documentation-only change.