npm-unpublish: add page

[Bucke200]

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The page(s) follow the style guide.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known):

[tldr-bot]

The build for this PR failed with the following error(s):

pages/common/npm-unpublish.md:5 MD032/blanks-around-lists Lists should be surrounded by blank lines [Context: "- Unpublish a specific package..."]
pages/common/npm-unpublish.md:5: TLDR007 Example descriptions should be surrounded by empty lines

Please fix the error(s) and push again.

[spageektti]

LGTM, thanks for your contribution