Some dev tooling and a small spelling fix

[lee0c]

Dev tooling

• Adds a requirements.txt file for easier pip install
• Updates to the sphinx config file to resolve errors during build
• Adds a gitignore file that ignores the default output folder from make

Spelling fix to address issue #60

• I can split this into its own PR if you want but figured I would grab it for a quick fix?
• Typo fix for categories_and_definitions page - only fixed in docs_source, not output files

[germonprez]

Thanks @lee0c! I think keeping this in a single PR is totally good :)