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

[emmadesilva]

For llms.txt, Resend does this well with an index like this

> ## Documentation Index
> Fetch the complete documentation index at: https://resend.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Send emails with Laravel

> Learn how to send your first email using Laravel.