getopt: add page

[xeruf]

Note that the page contains no placeholders, as it exists only to illustrate the usage.
None of the documented commands actually do anything useful by themselves.

• 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).

Version of the command being documented (if known):
2.37.2