Added a new captcha(cloudflare turnstile) and its corresponding
document. Cloudflare turnstile official instructions are here:
https://developers.cloudflare.com/turnstile
Signed-off-by: ByLCY <bylcy@bylcy.dev>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Jason Song <i@wolfogre.com>
