Matrix Authentication Service supports CAPTCHA protection (ReCaptcha
v2, Cloudflare Turnstile, hCaptcha) for certain operations like
self-service password registration, but configuring it required going
through matrix_authentication_service_configuration_extension_yaml.
Expose it via dedicated variables
(matrix_authentication_service_config_captcha_service, _site_key and
_secret_key), validate the service value and key presence, and document
the setup in the captcha documentation page, which so far only covered
Synapse and Dendrite.

Related to #5344

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>