run contentlayer build prior to next build in package.json

[jzxhuang]

See contentlayerdev/website#106, contentlayerdev/website#105

Putting the contentlayer build in package.json should make it easier for someone to clone this repo and deploy to an environment other than Gitpod

Note:

• I didn't actually check that this works on Gitpod, I'm hoping there will be CI for that
• It's probable that the build may fail due to Error when building with contentlayer 0.3.4 and Next.js 13 App Router contentlayer#506, but I suppose that's totally unrelated to this PR

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