As described here, with subst you can map a drive to a local folder temporarily (which means, after a reboot it is gone).
But there is another way to create a permanent drive mapped to a local folder.
Right-click to create a new text file (let's name it MapDriveD.txt).
Paste the following into it (Note that the first line must be exactly Windows Registry Editor Version 5.00 followed by a blank line):
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\DOS Devices]
"D:"="\??\C:\Users\myWinUser\Documents"
The local Windows user name (here: myWinUser) and its related path C:\Users\myWinUser\Documents need to exist.
Then, rename .txt to .reg and finally run MapDriveD.reg with elevated (administrator) rights, for example from an administrator command prompt.
After a reboot, you will have a permanent drive D: which points diretly into C:\Users\myWinUser\Documents.
Note: Of course, if you remove \\Documents in the .reg file, you can also map drive D: to C:\Users\myWinUser, and then you have the path D:\Documents you asked for.