Add prerendering security and privacy considerations to the spec

[kjmcnee]

The prerendering spec currently doesn't have sections for security and privacy considerations.
https://wicg.github.io/nav-speculation/prerendering.html

Several explainer documents in this repo describe such considerations, which should be added to the spec.

[caraitto]

Hi -- could we address this now that the I2S has been sent [0]? Thanks :)

[0] https://groups.google.com/a/chromium.org/g/blink-dev/c/n_OnfEwd8s0/m/5rZTtRw9BAAJ

[kjmcnee]

In #321 I've added the beginnings of the considerations sections in the prerendering spec which is for now just referencing the corresponding considerations in the related specs. That should address the NVS intent mentioned above.

These sections still need elaboration on prerendering itself.