Add tag taxonomy to blog example

[lucperkins]

This PR adds a tag taxonomy to the blog example. Specifically, it adds a tags field to the Post type and a /tags/[tag] endpoint.

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[lucperkins]

@seancdavis Okay, I think I've officially had too much fun with this 😄

[seancdavis]

@lucperkins How do you feel about splitting this off into a separate example? That way the example can stay focused, and our primary example can stay super simple.

[lucperkins]

Works for me! I'll get that cooking pretty soon here.

[lucperkins]

@seancdavis I've got this working locally. Were you thinking that I should keep this example under my GitHub org and just have it as a personal project or would you like to it have it under the CL org?

[seancdavis]

@lucperkins I'd say keep it under your user for now. But you can then open up a PR in contentlayerdev/website to add the example to the /examples page.