Add vector assign example and code

[cjemerson]

• Added assign description and example code to vector.md.
• Added assign.cpp in vector/ as a simple example.

[cjemerson]

I did not change the links in the drop down. I figured that you would want to change them in bulk.

[Bhupesh-V]

Hey @cjemerson the code looks fine 👍
Can you link the runnable code link instead of the local code (see issue #36 )
Also, add the whole source code in the example