Add/update I2P demo with deep dive video

@joshlambert has a great new deep dive video (https://www.youtube.com/watch?v=pBe4t1CD8Fc, starting at 3:16). We should add this to the I2P demo page (https://about.gitlab.com/handbook/product/i2p-demo). It shouldn't replace the existing short video, which is a bit more compelling for someone wanting an overview, but it's the only video that covers the new deep dive section. It also covers buying a license, which is out of scope of the I2P page. We could deep link to the deep dive section, and include the video near that section in the page. Or possibly just include two videos at the top. Not sure.

/cc @joshlambert @amara