feat: About us page

[blendthink]

close #24

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[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 893166e):

https://flutterkakgi-2023-conf-app-dev--pr85-feature-about-us-vi75trmc.web.app

_{(expires Fri, 01 Sep 2023 07:35:06 GMT)}

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_{Sign: 1cac8c9fbcf8c2f43fea34942176b864ab92a752}

[blendthink]

@FlutterKaigi/conference-app-2023 レビューをお願いします！

[blendthink]

レビューありがとうございます！ マージさせていただきます！