Improve Makefile Based On Feedback From @saivann

Make various improvements to the Makefile based on feedback from
@saivann.  (Thanks!) Improvements include:

* Add JEKYLL_COMMAND variable for users of older versions of Jekyll.
  Variable gets its value from: make invocation > environment > default
  value ("jekyll build")

* Remove an extraneous variable leftover from previous version.

* Add pre-build-checks-fast for checks which can be performed before the
  build, possibly saving the user debugging time

* Only check for missing images in the img/ subdirectory.

* Check for non-ascii URLs in translation template files.

* Describe how to use `make` in the README.