Release v1.2.0 - #596

Summary

• Bumps version to 1.2.0
• Updates NEWS.md with all changes since v1.1.2

This PR closes #593

Changes since v1.1.2

• Made add_delays() more flexible with configurable delay distributions and hospitalisation probability
• Added save_warmup argument to nfidd_sample()
• Fixed Stan function extraction regex to handle array[] return types
• Fixed forecasting model to generate forecast random walk in generated quantities
• Allowed zero-day delays in censored delay model
• Added forecasting to the joint nowcasting with R model
• Improved priors in joint nowcasting models
• Added Stan reference documentation pages
• Updated session content with improvements from SISMID

Checks

• devtools::test(): all passing
• devtools::check(): 0 errors, 1 warning (pre-existing executable Stan files), 3 notes (all pre-existing)