Docs: Add isatty(3) man page

Base/usr/share/man/man3/isatty.md

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+## Name
+
+isatty - check if a file descriptor is a TTY
+
+## Synopsis
+
+```**c++
+#include <unistd.h>
+
+int isatty(int fd);
+```
+
+## Description
+
+Checks if the device inside a given file descriptor is a TTY device.
+
+## Return value
+
+If `fd` refers to a TTY device, returns 1. Otherwise, returns 0 and `errno` is set to describe the error.
+
+## Errors
+
+* `EBADF`: `fd` is not an open file descriptor.
+* `ENOTTY`: `fd` refers to something that's not a TTY device.