Security page improvements

[20k-ultra]

document security implications for balenaMachine (you host it so it can be as secure as you make it!)
document summary of last time we did a pen test and explain how they (customer) can request access to the full report

[vipulgupta2048]

Some more issues that could be potential improvements?

• Indicate clearly what traffic goes over the VPN and what isn't #150
• User would like to know balenaCloud backup strategy for disaster recovery purposes #1282
• And, document the bug/threat reporting program