spec-kit docs for quickstart missing constitution definition

[mischadiehm]

https://github.github.com/spec-kit/quickstart.html

there is no initial hint to actually run: /speckit.constitution first. Add an example section that actually states how and why to use the constituition as e.g. described here: https://den.dev/blog/github-spec-kit/