Add maintenance and decommissioning stages

[duncandewhurst]

Related issues

• Closes Update project definition to cover maintenance and decommissioning #394
• Closes Update project status codelist and getting started documentation to incorporate maintenance and decommissioning #391
• Closes Add specific fields for project stage start and end dates #252
• Closes Project preparation period and approval period #421

Merge checklist

• Log your changes

If there are changes to project-schema.json or project-package-schema.json:

• Update the examples:
  • docs/examples/example.json
  • docs/examples/blank.json
• Run ./manage.py pre-commit to update docs/_static/i8n.csv

If you added a new definition to the schema, run ./manage.py pre-commit.

If you added a new codelist:

• Add an entry to docs/reference/codelists.md

[odscjen]

added a couple of full stops, otherwise looks good to me and ready to be passed to James for his review once the full stops have been added

[duncandewhurst]

I tidied up the changelog whilst resolving merge conflicts. Feel free to make further edits.

[jpmckinney]

Just one cleanup, then feel free to merge.