docs: #3419862 Explain form IDs and the Skip CAPTCHA permission

Jakob Prequested to merge
issue/captcha-3419862:3419862-cant-get-it into 2.x

Closes #3419862

Documentation only. The reporter's CAPTCHA was hidden by the "Skip CAPTCHA" permission, which works as designed, but the docs did not explain it or how to find a form ID.

  • Rewrites the README Configuration section around CAPTCHA points and "Add CAPTCHA challenges on all forms" (it still described a per-form challenge list that no longer exists).
  • Adds "Finding a form ID" (with contact form examples and the "Place a CAPTCHA here for untrusted users" admin links) and "Why don't I see the CAPTCHA?" (Skip CAPTCHA permission, IP whitelist, $settings['disable_captcha']).
  • Replaces the Form ID field description on the CAPTCHA point form with examples and the same pointer.

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