amp-ad: Fix capirs ad initialisation 🐛

[pyxarez]

As mentioned in the docs, we should put ad in div[id="c"] container in order to avoid bugs.
That pr fixes appending to the body.

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[pyxarez]

I signed it!

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[pyxarez]

I signed it!

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CLAs look good, thanks!

[pyxarez]

Hey! Is there someone to review PR:)

[ithinkihaveacat]

@pyxarez The link to the docs doesn't work for me, is it correct? i.e. https://jira.rambler-co.ru/browse/SUP-36063.

[pyxarez]

@ithinkihaveacat, yea sorry that was a wrong link, but I just fixed it

[pyxarez]

I mean that part of documentation https://github.com/ampproject/amphtml/blob/master/ads/README.md#the-iframe-sandbox