flatpak-{mask, update, remote-info}: add page

[Renner0E]

• 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):

My main goal was to document the process of rolling back to a known good version, finding out the commit id of good version and preventing the app from updating to the bad version again.