Add a page for mission and vision

[d-stahl-ericsson]

Description

It is customary for projects to state their mission and vision. We have only done so implicitly, but should do it explicitly. This is raised in the community repo in eiffel-community/community#10

This mission and vision should not only be in a .md somewhere, but on the github.io page.

Motivation

To clarify the community's mission and vision.

Exemplification

Benefits

Greater visibility of the community's mission and vision.

Possible Drawbacks

One argument against would be that the github.io page might become cluttered if we add everything and kitchen sink to it.