doc: mainpage: fix dead Gmane links

Gmane is sadly offline since 2016, so let's link to our own (not so
nice) archive for now, so people can still at least crawl through it.

doc/doxygen/src/mainpage.md

@@ -32,14 +32,12 @@ RIOT is developed by an open community that anyone is welcome to join:
    Wiki](https://github.com/RIOT-OS/RIOT/wiki/Contributing-to-RIOT).
  - [Subscribe](http://lists.riot-os.org/mailman/listinfo/users) to
    users@riot-os.org to ask for help using RIOT or writing an application for
-   RIOT (or to just stay in the loop). A searchable archive of this list is
-   available at the
-   [RIOT user Gmane newsgroup](http://news.gmane.org/gmane.os.riot.user)
+   RIOT (or to just stay in the loop). An archive of this list [is available
+   here](https://lists.riot-os.org/pipermail/users/).
  - [Subscribe](http://lists.riot-os.org/mailman/listinfo/devel) to
    devel@riot-os.org to follow and discuss kernel and network stack
-   developement, or hardware support. A searchable archive of this list is
-   available at the
-   [RIOT devel Gmane newsgroup](http://news.gmane.org/gmane.os.riot.devel)
+   developement, or hardware support. An archive of this list [is available
+   here](https://lists.riot-os.org/pipermail/devel/).
  - Follow us on [Twitter](https://twitter.com/RIOT_OS) for news from the RIOT
    community.
  - Regarding critical vulnerabilities we would appreciate if you give us a