[Release][Docs] Normalize previous document version directory

We should use X.Y instead of X.Y.Z (e.g.: 8.0 not 8.0.1) for previous version document directory.

See also: https://github.com/apache/arrow/blob/apache-arrow-9.0.0/dev/release/post-08-docs.sh#L84

The script should accept X.Y.Z such as 8.0.1 and normalize it to X.Y. It'll reduce human error.

See also:

• https://github.com/apache/arrow-site/pull/228#issuecomment-1205997067
• https://github.com/apache/arrow-site/pull/228#issuecomment-1206085602