update extensions.md, Namespace section

Make clear that "type" in extensions' namespaces must be in plural form (widgets/behaviors/modules)
fixed invalid upstream url
Merge pull request #1928 from tomtomsen/invalid_remote_url_in_workflow_md

workflow.md: fixed invalid upstream url
Merge pull request #1927 from artem-frolov/update-namespace-section-in-extensions-doc

Update extensions.md, Namespace section