Copy .txt files from the _pages directory to the _site output during build

[emmadesilva]

Think a build task would be good for this. For example adding robots.txt or llms.txt. It should support nested paths too.

So _pages/foo.txt => _site/foo.txt
And _pages/foo/bar.txt => _site/foo/bar.txt

I don't think we need a config/feature flag for this since I don't think it has any unexpected side effects because there would be no reason to put a TXT file in the _pages directory as Hyde wouldn't do anything with it.

This would also resolve hydephp/hyde#255