docs: update the url of Ninja web site
docs: update the url of Ninja web site

This patch updates the url of Ninja to point to the new one that is now
https://ninja-build.org/.

BUG=None
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/1730173005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#377847}

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