docs: Rename esp_pthread docs to pthread, expand details of supported APIs

- Move the code that was in the document out to an example

examples/system/pthread/main/pthread_example.c

@@ -0,0 +1,67 @@
+/* Pthread Example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+#include <stdio.h>
+#include <unistd.h>
+#include <pthread.h>
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+#include "esp_pthread.h"
+
+static void *example_thread(void * arg);
+
+void app_main(void)
+{
+    pthread_attr_t attr;
+    pthread_t thread1, thread2;
+    esp_pthread_cfg_t esp_pthread_cfg;
+    int res;
+
+    // Create a pthread with the default parameters
+    res = pthread_create(&thread1, NULL, example_thread, NULL);
+    assert(res == 0);
+    printf("Created thread 0x%x\n", thread1);
+
+    // Create a pthread with a larger stack size using the standard API
+    res = pthread_attr_init(&attr);
+    assert(res == 0);
+    pthread_attr_setstacksize(&attr, 16384);
+    res = pthread_create(&thread2, &attr, example_thread, NULL);
+    assert(res == 0);
+    printf("Created larger stack thread 0x%x\n", thread2);
+
+    res = pthread_join(thread1, NULL);
+    assert(res == 0);
+    res = pthread_join(thread2, NULL);
+    assert(res == 0);
+    printf("Threads have exited\n\n");
+
+    // Use the ESP-IDF API to change the default thread attributes
+    esp_pthread_cfg = esp_pthread_get_default_config();
+    esp_pthread_cfg.stack_size = 32768;
+    esp_pthread_cfg.prio += 2;
+    ESP_ERROR_CHECK( esp_pthread_set_cfg(&esp_pthread_cfg) );
+
+    res = pthread_create(&thread1, NULL, example_thread, NULL);
+    assert(res == 0);
+    printf("Created thread 0x%x with new default config\n", thread1);
+    res = pthread_join(thread1, NULL);
+    assert(res == 0);
+    printf("Thread has exited\n\n");
+}
+
+static void *example_thread(void * arg)
+{
+    usleep(250 * 1000);
+    printf("This thread has ID 0x%x and %u bytes free stack\n", pthread_self(), uxTaskGetStackHighWaterMark(NULL));
+
+    sleep(1);
+    printf("Thread 0x%x exiting\n", pthread_self());
+
+    return NULL;
+}