Extend Makefile with "deps", "dev", and "prod" targets

The "make deps" target downloads 3rd-party JavaScript resources that
are used by the site, (currently React and React-Dom). The "make dev"
installs the development versions locally while "made prod" installs
the production versions.

All three targets verify the integrity of the downloaded files via
SHA512 sums that are added in this commit.

And all three targets are documented in the README file added in this
commit.