Include sample template SSP generation in web docs

[danielnaab]

Extended Description
As a user drafting a FedRAMP OSCAL SSP, I would like a template document that I can use as a starting point.

Context: a transform exists to generate a sample SSP. This transform could either be better documented, and/or included in the web interface. See: https://github.com/18F/fedramp-automation/blob/master/src/validations/rules/sample-ssp.xsl

Acceptance Criteria

• Sample SSP generation is accessible to end-users

Definition of Done

• Acceptance criteria met
• Unit test coverage of our code > 90% - needs automation story
• Accessibility tests pass - needs automation story
• Automated code quality checks pass
• Security reviewed and reported - check in with Wes on what we could do here
• Reviewed against plain language guidelines
• Design QA passed
• Code must be self-documenting
• No local tech debt
• Documentation updated
• Architectural Decision Record completed as necessary for significant design choices
• PR reviewed & approved
• Source code merged