Merge #19390 #19391

19390: doxygen: Point to contributing.md document r=kaspar030 a=bergzand

### Contribution description

Shortcuts a referral on the wiki


### Testing procedure

Check the url in the doxygen main page

### Issues/PRs references

None

19391: cord: convert to ztimer r=kaspar030 a=bergzand

### Contribution description

As the subject describes


### Testing procedure

The `examples/cord_ep` example should work as before.


### Issues/PRs references

None


Co-authored-by: Koen Zandberg <koen@bergzand.net>

doc/doxygen/src/mainpage.md

@@ -33,8 +33,8 @@ Contribute to RIOT                                        {#contribute-to-riot}
 RIOT is developed by an open community that anyone is welcome to join:
  - Download and contribute your code on
    [GitHub](https://github.com/RIOT-OS/RIOT). You can read about how to
-   contribute [in our GitHub
-   Wiki](https://github.com/RIOT-OS/RIOT/wiki/Contributing-to-RIOT).
+   contribute [in our contributing
+   document](https://github.com/RIOT-OS/RIOT/blob/master/CONTRIBUTING.md).
  - Sign-up to our [forum](https://forum.riot-os.org/) to ask for help using RIOT
    or writing an application for RIOT, discuss kernel and network stack
    development as well as hardware support, or to show-case your latest project.

sys/Makefile.dep

@@ -810,7 +810,7 @@ endif
 
 ifneq (,$(filter cord_ep_standalone,$(USEMODULE)))
   USEMODULE += cord_ep
-  USEMODULE += xtimer
+  USEMODULE += ztimer_msec
 endif
 
 ifneq (,$(filter cord_lc,$(USEMODULE)))

sys/net/application_layer/cord/ep/cord_ep_standalone.c

@@ -23,7 +23,7 @@
 #include "log.h"
 #include "assert.h"
 #include "thread.h"
-#include "xtimer.h"
+#include "ztimer.h"
 #include "net/cord/ep.h"
 #include "net/cord/config.h"
 #include "net/cord/ep_standalone.h"
@@ -38,11 +38,11 @@
 
 #define UPDATE_TIMEOUT      (0xe537)
 
-#define TIMEOUT_US          ((uint64_t)(CONFIG_CORD_UPDATE_INTERVAL * US_PER_SEC))
+#define TIMEOUT_MS          (CONFIG_CORD_UPDATE_INTERVAL * MS_PER_SEC)
 
 static char _stack[STACKSIZE];
 
-static xtimer_t _timer;
+static ztimer_t _timer;
 static kernel_pid_t _runner_pid;
 static msg_t _msg;
 
@@ -50,7 +50,7 @@ static cord_ep_standalone_cb_t _cb = NULL;
 
 static void _set_timer(void)
 {
-    xtimer_set_msg64(&_timer, TIMEOUT_US, &_msg, _runner_pid);
+    ztimer_set_msg(ZTIMER_MSEC, &_timer, TIMEOUT_MS, &_msg, _runner_pid);
 }
 
 static void _notify(cord_ep_standalone_event_t event)
@@ -94,7 +94,7 @@ void cord_ep_standalone_run(void)
 void cord_ep_standalone_signal(bool connected)
 {
     /* clear timer in any case */
-    xtimer_remove(&_timer);
+    ztimer_remove(ZTIMER_MSEC, &_timer);
     /* reset the update timer in case a connection was established or updated */
     if (connected) {
         _set_timer();