content: add documentation of virtual properties

[brandyscarney]

URL

https://ionicframework.com/docs/core-concepts/fundamentals

Issue Description

There is currently no documentation on how virtual properties work in the Ionic Framework.

We should add documentation similar to the following:

Virtual properties do not behave the same as @Prop decorated properties. They are not reactive and are not intended to be. They will not take effect if mutated and will not cause a re-render of the component.

Related issue: ionic-team/ionic-framework#29167

The documentation should be added at the following URL: https://ionicframework.com/docs/core-concepts/fundamentals

A section called Properties, with a sub-section titled Virtual Properties, should be added.