ESP8266WiFi library documentation (#2388)

* New section with ESP8266WiFi library documentation

* ESP8266WiFi library documentation

1. Introduction - example, diagnostic, doxygen
2. Station - examples, new doc
3. Soft Access Point - examples, new doc
4. Scan - examples, new doc
5. Client - examples, ref. Arduino, setNoDelay, list of functions
6. Client Secure - examples, loadCertificate, setCertificate, list of
functions
7. Server- examples, ref. Arduino, setNoDelay, list of functions
8. UDP - examples, ref. Arduino, Multicast UDP
9. Generic - examples, onEvent, WiFiEventHandler, persistent, mode, list
of functions

* Fixed numbered list

doc/esp8266wifi/client-secure-class.md

@@ -0,0 +1,81 @@
+---
+title: ESP8266WiFi Client Secure Class
+---
+
+[ESP8266WiFi Library :back:](readme.md#client-secure)
+
+
+## Client Secure Class
+
+Methods and properties described in this section are specific to ESP8266. They are not covered in [Arduino WiFi library](https://www.arduino.cc/en/Reference/WiFi) documentation. Before they are fully documented please refer to information below.
+
+
+### loadCertificate
+
+Load client certificate from file system.
+
+```cpp
+loadCertificate(file) 
+```
+
+*Declarations*
+```cpp
+#include <FS.h>
+#include <ESP8266WiFi.h>
+#include <WiFiClientSecure.h>
+
+const char* certyficateFile = "/client.cer";
+```
+
+*setup() or loop()*
+```cpp
+if (!SPIFFS.begin()) 
+{
+  Serial.println("Failed to mount the file system");
+  return;
+}
+
+Serial.printf("Opening %s", certyficateFile);
+File crtFile = SPIFFS.open(certyficateFile, "r");
+if (!crtFile)
+{
+  Serial.println(" Failed!");
+}
+
+WiFiClientSecure client;
+
+Serial.print("Loading %s", certyficateFile);
+if (!client.loadCertificate(crtFile))
+{
+  Serial.println(" Failed!");
+}
+
+// proceed with connecting of client to the host
+```
+
+
+### setCertificate
+
+Load client certificate from C array.
+
+```cpp
+setCertificate (array, size) 
+```
+
+For a practical example please check [this interesting blog](https://nofurtherquestions.wordpress.com/2016/03/14/making-an-esp8266-web-accessible/).
+
+
+### Other Function Calls
+
+```cpp
+bool  verify (const char *fingerprint, const char *domain_name) 
+void  setPrivateKey (const uint8_t *pk, size_t size) 
+bool  loadCertificate (Stream &stream, size_t size) 
+bool  loadPrivateKey (Stream &stream, size_t size) 
+template<typename TFile >  bool  loadPrivateKey (TFile &file)
+```
+
+Documentation for the above functions is not yet prepared.
+
+
+For code samples please refer to separate section with [examples :arrow_right:](client-secure-examples.md) dedicated specifically to the Client Secure Class.