Add PyYAML dependency to docs and setup.py

[EdwardLarson]

OFRAK has PyYAML dependencies in three places which are not explicit in code or documentation:

• ofrak_ghidra requires it
• build_image.py requires it for the Docker build system
• Building the OFRAK docs requires PyYAML

These three dependencies should be handled separately:

• ofrak_ghidra's setup.py shall include PyYAML
• INSTALL.md will explain that PyYAML is required for the Docker build
• PyYAML will be included in the docs requirements in the extras_require section for the core ofrak package.

Originally posted by @joshterrill in #6 (comment)