Get rid of the 'Technical Specs' page and add some technical info in 'About'

This was briefly discussed in #monero-research-lab and #monero-site. The page is outdated and has a dedicated submenu, but at the end do we really need it? Better add some technical details in the About page; like block time, POW, emission and privacy enhancing technologies at the base of Monero. Feel free to discuss what to keep and what to remove.