Fails to build on Windows when docs drive is different from TEMP

When building documentation under Windows, fails with the message like:
> Error reading page 'index.md': path is on mount 'C:', start on mount 'T:'

The reason, as I understand, is monorepo plugin creating some temporary files there, and path.relpath() throwing that as an exception.

Reproduce the documentation build on Windows with the documentation drive different from TEMP, then trace the monorepo plugin's temporary-file handling to the path.relpath() call mentioned in the report. Done means the build completes successfully across different drive letters without the mount-related exception.

Written by the indexing model from the issue text.