HPCC-9514 Add initial documentation about roxiemem

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
HPCC-9514 Merge contents of sourcedoc.xml and remove it.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4514 from ghalliday/issue9514

HPCC-9514 Add initial documentation about roxiemem

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>