update docs to reflect changed names of nbgrader commands

[kcranston]

Revise the documentation in nbgrader_local_dev.md to reflect new names for some nbgrader commands (e.g. assign is now generate_assignment).

[kcranston]

List of changed commands from 0.6.0 release notes:

• nbgrader assign is now nbgrader generate_assignment
• nbgrader release is now nbgrader release_assignment
• nbgrader fetch is now nbgrader fetch_assignment

[kcranston]

Changed hackmd (only generate_assignment since we aren't using release and fetch functionality).