first steps in migration notes

[turbomam]

• Add documentation for the legacy MongoDB contents migrator #1060

[Shalsh23]

I have tried the currently listed first four steps, and they are working for me. I was able to setup a tunnel via dtn01.nersc.gov node to the production MongoDB server.

[turbomam]

Thanks @Shalsh23

I'm going to merge this, but let's remember to integrate it with whatever other documentation your and @eecavanna come up with

I would prefer not to leave it in assets/misc... maybe src/docs would be a better choice