apt-mark: add page (#2520)

pages/linux/apt-mark.md

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+# apt-mark
+
+> Utility to change the status of installed packages.
+
+- Mark a package as automatically installed:
+
+`sudo apt-mark auto {{package_name}}`
+
+- Hold a package at its current version and prevent updates to it:
+
+`sudo apt-mark hold {{package_name}}`
+
+- Allow a package to be updated again:
+
+`sudo apt-mark unhold {{package_name}}`
+
+- Show manually installed packages:
+
+`apt-mark showmanual`
+
+- Show held packages that aren't being updated:
+
+`apt-mark showhold`