fix: recursively clear docs before copy

[darad124]

Summary:

• Recursively clear docs before copying generated files.
• Copy build output synchronously so the script finishes after files are written.

Closes #11

Validation:

• Created a temporary nested build directory and ran node copy_to_docs.js.
• Confirmed it copied nested folders without the previous EISDIR crash.

Note: full Docusaurus build is blocked upstream because Docusaurus 3 expects react-dom/client while this package pins React 17.