Implement PoC of Makefile target documentation pattern

[eecavanna]

Example usage

$ make

(...)

cookiecutter-help: Show targets inherited from cookiecutter
status: Show project metadata
migration-doctests: Run migration-related doctests
migrator: Generate a migrator interactively

$

Screenshot of output:

[eecavanna]

Hi @turbomam, I have updated this branch to contain the additional #// ... documentation lines we came up with during today's hackathon.

Here's a screenshot of the current output of make help (minus the boilerplate content included at the top of the output):

[eecavanna]

I'm going to mark this as ready for review, since the "plumbing" is implemented in this PR branch. People can add the specific documentation comments in a separate PR.

[eecavanna]

To consider:

• Move the sed script to a dedicated make target (instead of piggybacking onto the help target, given the latter's baggage)
• Using something other than sed (e.g. a Python script)