fix(doc): fix disk-space leak (#3019)
**Summary**
Remedy disk-space leak where `partition_doc()` would leave a copy of
each `.doc` file passed as a file-like object on disk.
**Additional Context**
`partition_doc()` creates a temporary file in which it writes each
source-document provided as a file-like object. This file is not deleted
and disk consumption grows without bound.
The `convert_office_doc()` function used to convert DOC->DOCX uses a
command-line program provided with LibreOffice to convert do the
conversion. Because this command-line program operates in a different
memory space, the source file cannot be passed as an in-memory object
and needs to be on the filesystem. When the DOC file is passed as a
file-like object, it is written to disk so the conversion program has
access to it. It is not deleted afterward.
Fix this by writing the temporary source DOC file in the
TemporaryDirectory already being used to write the conversion-target
DOCX file. That directory is automatically removed when
`partition_doc()` completes.
