Add documentation for the Unit crock

[dalefrancis88]

Description

To bring it in-line with the other Crocks we need to add in descriptive documentation for the Unit crock with examples

Task List

• Add main Unit.md file to 'docs/src/pages/docs/crocks/' based on the others
• Ensure it has the correct 'weight'
• Locate all references to Unit and update it to link to the new Unit documentation. Reference the section where possible
• Update docs/src/pages/docs/crocks/index.md with links to the new Unit.md page
• Update docs/src/pages/docs/functions/pointfree-functions.md with links to the new Unit.md page

For more information, see the CONTRIBUTING guide.

Further Information

This was sourced from #42