common/*: update pages of POSIX special Built-In utilities (#11858)

* eval, exec, exit, export, readonly, set, shift, trap: use Open Group links

* exec, export, trap: split Linux and POSIX

pages/linux/export.md

@@ -0,0 +1,20 @@
+# export
+
+> Command to mark shell variables in the current environment to be exported with any newly forked child processes.
+> More information: <https://www.gnu.org/software/bash/manual/bash.html#index-export>.
+
+- Set a new environment variable:
+
+`export {{VARIABLE}}={{value}}`
+
+- Remove an environment variable:
+
+`export -n {{VARIABLE}}`
+
+- Mark a shell function for export:
+
+`export -f {{FUNCTION_NAME}}`
+
+- Append something to the PATH variable:
+
+`export PATH=$PATH:{{path/to/append}}`