docs: add access control and rate limiting guidance

docs: update registry major version references

docs: add production notes

docs: add v2 vs v3 notes

docs: align config path with registry v3

docs: upgrade registry image to 3.0.0

docs: remove roadmap closing line

docs: add toc and refine roadmap wording

docs: mark future enhancements

docs: add reference links

docs: add full nginx auth example

docs: use sample 35-char auth secret

docs: add Docker Hub signup link and proxy note

docs: clarify proxy prerequisite

docs: link guide in readme