Instruct to install imagemagick directly, without development packages

[vsalvino]

See issue #642

Confirmed pip install wand and >>> from wand.image import Image works correctly based on the update installation methods on Debian and Fedora.

Since the user of the wand python package is not expected to be actively compiling an imagemagick C library, the development dependencies are not necessary. Only the actual imagemagick end-user library.

[emcconville]

Thanks!

Please drop those Docker paragraphs below the Debian & Fedora installs.

If you're using in a Docker image, it may be beneficial to avoid installing
extra recommended packages:

The Docker image, beneficial and non-essential are opinions of a common virtualization use-case, and is not the roll of this Installation guide to comment on such. Best just keep everything generic. Dropping the devel dependencies are enough.

[vsalvino]

Happy to remove it, just wanted to share my reasoning below first.

As someone who works with juniors or newcomers frequently, in my experience most people using Docker do not know how to optimize for Docker, and have little understanding of Linux. Given that a huge percentage of Python is run in Docker images, I figured it would be beneficial to share this tidbit rather than assume the reader already knows the best installation method. The problem is compounded these days by Artificial Unintelligence that will encourage people to copy-paste stuff straight from the search results without thinking about it. By adding this content we can help steer the search engines in the right direction as well.