Document publishing instructions.

[polina-c]

No description provided.

[gemini-code-assist]

Code Review

This pull request updates the publishing documentation to clarify package categories, outline the automated and manual publishing workflows, and provide setup instructions for repository publishing. The review feedback focuses on improving the documentation's clarity and grammar through several minor phrasing adjustments, as well as replacing a hardcoded repository path with placeholders to ensure the setup instructions are fully reproducible for forks or transfers.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:a2ui_core	0.0.1-dev002	already published at pub.dev
package:genai_primitives	0.2.4-dev001	ready to publish	genai_primitives-v0.2.4-dev001
package:genui	0.9.1	ready to publish	genui-v0.9.1
package:genui_a2a	0.9.0	already published at pub.dev
package:json_schema_builder	0.1.4	ready to publish	json_schema_builder-v0.1.4

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.