Add Validator Election Page

[joepetrowski]

Add a page about how validator set changes take place. This new page should document:

• Why the Phragmen election is performed off chain
• Side effects/considerations (e.g. limited function availability)
• How the election results take effect

[JoshOrndorff]

IMO it is important that articles like this state the assumptions at the beginning. Not all substrate chains will have staking or even sessions, so before we start explaining how staking works we should make it clear that the docs are not relevant to all chains, and how you can tell whether they are relevant to your chain.