Parent ticket: Documentation revisions/additions for add-ons

[stichbury]

Add-ons introduce changes to the way Kedro projects can be created. There are some changes to the docs required to update what we have and add new guidance. This is a parent ticket and I'll create a set of children for activities related to this.

• Updates to docs that use the Iris starter which we are sunsetting
• Updates to starters docs (in framework repo) to deprecate some and add pyspark spaceflights
• Update documentation to reflect the new kedro new add-ons flow#3048
• A blog post
• Updates to starters READMEs (not related to add-ons but they're inconsistent at present)

[stichbury]

This can be closed as the only leftover is a blog post.