Add generator documentation

[michaellans]

• Adds generators directory within documentation/docs/guides/ with .md files giving individual algorithm overviews and xopt generator parameters. These files are also used for building generator docs via the BadgerDocsWindow within the GUI
• updated factory.get_generator_docs() to combine Badger documentation .md file with xopt generator docstring, similar to the environment docs
• Fixed typo in help_md string construction to only show the docstring as a code block and render the rest as markdown