pg_dumpall: add page

[deed02392]

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

Version of the command being documented (if known):
14