samples: posix: use zephyr:code-sample directive

Describe the samples using code-sample directive in preparation for
upcoming changes to the Zephyr documentation that will be leveraging
the provided description and metadata.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

samples/posix/env/README.rst

@@ -1,7 +1,7 @@
-.. _posix-env-sample:
+.. zephyr:code-sample:: posix-env
+   :name: Environment Variables
 
-POSIX Environment Variables
+   Manipulate environment variables from a Zephyr application.
-###########################
 
 Overview
 ********

samples/posix/eventfd/README.rst

@@ -1,7 +1,7 @@
-.. _posix-eventfd-sample:
+.. zephyr:code-sample:: posix-eventfd
+   :name: eventfd()
 
-POSIX eventfd()
+   Use ``eventfd()`` to create a file descriptor for event notification.
-###############
 
 Overview
 ********

samples/posix/gettimeofday/README.rst

@@ -1,7 +1,7 @@
-.. _posix-gettimeofday-sample:
+.. zephyr:code-sample:: posix-gettimeofday
+   :name: gettimeofday() with clock initialization
 
-POSIX gettimeofday() with clock initialization over SNTP
+   Use ``gettimeofday()`` with clock initialization over SNTP.
-########################################################
 
 Overview
 ********

samples/posix/philosophers/README.rst

@@ -1,7 +1,7 @@
-.. _posix-philosophers-sample:
+.. zephyr:code-sample:: posix-philosophers
+   :name: POSIX Philosophers
 
-POSIX Philosophers
+   Implement a solution to the Dining Philosophers problem using the POSIX API.
-##################
 
 Overview
 ********

samples/posix/uname/README.rst

@@ -1,7 +1,7 @@
-.. _posix-uname-sample:
+.. zephyr:code-sample:: posix-uname
+   :name: uname()
 
-POSIX uname()
+   Use ``uname()`` to acquire system information and output it to the console.
-#############
 
 Overview
 ********