Base: Add more manpages for command-line utilities

Base/usr/share/man/man1/cat.md

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+## Name
+
+cat - concatenate files to stdout
+
+## Synopsis
+
+```**sh
+$ cat [file...]
+```
+
+## Description
+
+This program passes contents of specified `files` to standard output, in the specified order. If no `file` is specified, or it is `-`, it defaults to standard input.
+
+## Arguments
+
+* `file`: Files to print
+
+## Examples
+
+```sh
+# Display a single file
+$ cat README.md
+# SerenityOS
+
+Graphical Unix-like operating system for x86 computers.
+...
+
+# Display standard input
+$ cat
+aaa
+aaa
+bbb
+bbb^C
+
+# Display ls output where each file is in separate line
+$ ls | cat
+Desktop
+Documents
+Downloads
+README.md
+Source
+js-tests
+tests
+web-tests
+
+# Display multiple files
+$ echo 123 > test.txt
+$ echo 456 > test2.txt
+cat test.txt test2.txt
+123
+456
+```