This directory is Material's `custom_dir`.

The build stages the shared theme overrides here with no-clobber, so it is
normally empty in a project repo and everything is inherited.

To customise one thing for this repo only, commit a file with the same path
as the shared one and it will win. The shared files live in
`design-system/overrides/` in willtheorangeguy/mkdocs.

Do not delete this directory: `theme.custom_dir` points at it and MkDocs
warns when it is missing.
