Create fulcio-spec.md (#3)

* Create fulcio-spec.md

Move spec from Google doc to Github

Signed-off-by: Tracy Miranda <tracymiranda@users.noreply.github.com>

* Add OIDC link & format fixes

Signed-off-by: Tracy Miranda <tracymiranda@users.noreply.github.com>

* Terminology fixes for clarity

Signed-off-by: Tracy Miranda <tracymiranda@users.noreply.github.com>

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Signed-off-by: Tracy Miranda <tracymiranda@users.noreply.github.com>