Create a symbolic link do navigate with cd
The problem with creating a symbolic link in that manner is that you are limited to where the symbolic link is created.  It also adds to the problem if you have a directory ../d/.. somewhere on your system.
There are a couple of ways to solve this....
Create an Alias
In terminal, you can create an alias by issuing the command
alias cdd='cd ~/Documents'
The benefit here is that it will work anywhere without having to add it to your PATH.
To make it permanent, add the command to your .bash_profile in your home directory.
Make a variable cd-able
Also, in your .bash_profile add the following
shopt -s cdable_vars 
export Docs=$HOME/Documents

Now, when you type cd Docs it will expand out HOME to whatever your user directory is plus the directory you specified. (/Users/yourusername/Documents)
Personally, I did this with a variable called icloud as follows:
export  icloud=$HOME/Library/Mobile\ Documents/com~apple~CloudDocs/

Now, I can reference my iCloud documents location without having to remember that lengthy path.