pyproject: add API documentation link to metadata (#424)

* pyproject: add API documentation link to metadata

Signed-off-by: William Woodruff <william@trailofbits.com>

* Update pyproject.toml

Co-authored-by: Dustin Ingram <di@users.noreply.github.com>
Signed-off-by: William Woodruff <william@yossarian.net>

Signed-off-by: William Woodruff <william@trailofbits.com>
Signed-off-by: William Woodruff <william@yossarian.net>
Co-authored-by: Dustin Ingram <di@users.noreply.github.com>

Author: woodruffw

pyproject.toml

@@ -45,6 +45,7 @@ sigstore = "sigstore._cli:main"
 Homepage = "https://pypi.org/project/sigstore/"
 Issues = "https://github.com/sigstore/sigstore-python/issues"
 Source = "https://github.com/sigstore/sigstore-python"
+Documentation = "https://sigstore.github.io/sigstore-python/"
 
 [project.optional-dependencies]
 test = [