[CAPTCHA] new material in the introduction

As agreed at last week’s meeting, I added material describing the organizing themes of the document.
The pull request can be reviewed here:
https://github.com/w3c/captcha-accessibility/pull/44

and the proposed text is at
https://raw.githack.com/w3c/captcha-accessibility/main-themes/index.html

Comments are welcome. I can revise other sections if additional need for change is identified.


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