az-pipelines: add page

[salem84]

• The page (if new), does not already exist in the repository.
• The page is in the correct platform directory (common/, linux/, etc.)
• The page has 8 or fewer examples.
• The PR title conforms to the recommended templates.
• The page follows the content guidelines.
• The page description includes a link to documentation or a homepage (if applicable).

For: #6141

This command (but there are many others in az devops CLI scope) has many subcommands (such as az pipelines agent, az pipelines pool...)
Do we create other pages for these?
Thanks

[marchersimon]

Thanks @salem84