Deployment documentation for the Landing Page

• Include deployment documentation in the Landing Page's README.md and/or docs/ folder.
• Document the simpler procedure (user forks Onion Launchpad's mirror on GitLab or GitHub, then configure and trigger a build). Moved to #78.
• Document logo filename convention for automatic translations.
• Include a screenshot.
• Explain that it's useful both as a censorship circumvention tool and as a landing portal for onion-only sites that needs to include documentation for their users about how to get access.