help: add page (#10547)

* help: add page


---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/linux/help.md

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+# help
+
+> Display information about Bash builtin commands.
+> More information: <https://www.gnu.org/software/bash/manual/bash.html#index-help>.
+
+- Display the full list of builtin commands:
+
+`help`
+
+- Print instructions on how to use the `while` loop construct:
+
+`help while`
+
+- Print instructions on how to use the `for` loop construct:
+
+`help for`
+
+- Print instructions on how to use `[[ ]]` for conditional commands:
+
+`help [[ ]]`
+
+- Print instruction on how to use `(( ))` to evaluate arithmetic expressions:
+
+`help \( \)`
+
+- Print instructions on how to use the `cd` command:
+
+`help cd`