gofumpt: add page

[TheRootDaemon]

Checklist

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• 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 contains at most 5 new pages.
• The PR is authored by me, or has been human-reviewed if it was created with AI or machine translation software.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known):
• Reference issue: #

[emmanuel-ferdman]

I think the --extra flag should be added as it's what differentiates gofumpt from gofmt - it enables additional strict formatting rules.