Auto-generate _headers file

[lucperkins]

At the moment, Netlify deployment includes copying a file from the root into the public folder. This PR introduces changes that generate the _headers file from a template. Right now the template doesn't include any special logic (it's essentially just the original text file) but this setup will enable us to generate a more fine-grained _headers file in the future.

Testing

I've tested this locally via the following steps:

$ rm -rf public
$ hugo
$ cat public/_headers
/*
  X-Robots-Tag: noindex

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[lucperkins]

I'm going to close this PR in favor of #9225, which includes these changes but is more comprehensive.