[Performance] Copying files with `utils::fs::copy_files_except_ext` is slow

Problem

Our book is growing and by now building it takes approx. 1m30s. Most of the time is spent in HtmlHandlebars::render when copying files from src to dest.

In a small proof-of-concept I replaced the call to copy_files_except_ext locally with rsync. Execution time dropped to 8s.

Proposed Solution

The copy_files_except_ext seems to be a bottleneck. Various performance improvements can be thought of:

• Parallelize, e.g., by starting threads for copying sub-directories (though not trivial to fine-tune for performance, e.g., limit to CPU cores or the like,...)
• Use platform-dependent tools like rsync if available. Very fast, allows for exclude patterns.

Notes

I've seen the comment on recursion while copying. I did not dig into that but is it really an issue? Only if the target directory is a sub-directory of source or if we encounter a (sym-)link into the target directory somewhere in source, I would assume. Both would be strange setups, wouldn't it?

Start by locating utils::fs::copy_files_except_ext and its use from HtmlHandlebars::render. Reproduce the reported build timing, compare it with the rsync proof of concept, and investigate the existing recursion comment. Done means file copying is measurably faster without breaking the stated source-to-destination behavior.