doc: mainpage: fix dead Gmane links
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.

cladmi/RIOT,kbumsik/RIOT,toonst/RIOT,yogo1212/RIOT,toonst/RIOT,josephnoir/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,aeneby/RIOT,A-Paul/RIOT,kYc0o/RIOT,ant9000/RIOT,rfuentess/RIOT,yogo1212/RIOT,biboc/RIOT,OTAkeys/RIOT,authmillenon/RIOT,avmelnikoff/RIOT,ant9000/RIOT,mtausig/RIOT,kYc0o/RIOT,neiljay/RIOT,basilfx/RIOT,biboc/RIOT,rfuentess/RIOT,josephnoir/RIOT,jasonatran/RIOT,cladmi/RIOT,kaspar030/RIOT,miri64/RIOT,kYc0o/RIOT,OlegHahm/RIOT,jasonatran/RIOT,mfrey/RIOT,avmelnikoff/RIOT,ant9000/RIOT,aeneby/RIOT,avmelnikoff/RIOT,authmillenon/RIOT,josephnoir/RIOT,ant9000/RIOT,authmillenon/RIOT,biboc/RIOT,aeneby/RIOT,yogo1212/RIOT,yogo1212/RIOT,kbumsik/RIOT,neiljay/RIOT,mfrey/RIOT,kaspar030/RIOT,mtausig/RIOT,BytesGalore/RIOT,toonst/RIOT,jasonatran/RIOT,A-Paul/RIOT,neiljay/RIOT,basilfx/RIOT,x3ro/RIOT,mfrey/RIOT,A-Paul/RIOT,mtausig/RIOT,kaspar030/RIOT,lazytech-org/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,biboc/RIOT,josephnoir/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,lazytech-org/RIOT,OTAkeys/RIOT,cladmi/RIOT,neiljay/RIOT,avmelnikoff/RIOT,kaspar030/RIOT,gebart/RIOT,kYc0o/RIOT,BytesGalore/RIOT,authmillenon/RIOT,gebart/RIOT,kbumsik/RIOT,smlng/RIOT,gebart/RIOT,RIOT-OS/RIOT,basilfx/RIOT,BytesGalore/RIOT,aeneby/RIOT,cladmi/RIOT,yogo1212/RIOT,lazytech-org/RIOT,x3ro/RIOT,gebart/RIOT,gebart/RIOT,cladmi/RIOT,miri64/RIOT,BytesGalore/RIOT,mtausig/RIOT,jasonatran/RIOT,avmelnikoff/RIOT,miri64/RIOT,jasonatran/RIOT,mtausig/RIOT,smlng/RIOT,x3ro/RIOT,biboc/RIOT,josephnoir/RIOT,yogo1212/RIOT,A-Paul/RIOT,mfrey/RIOT,RIOT-OS/RIOT,basilfx/RIOT,kaspar030/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,rfuentess/RIOT,authmillenon/RIOT,miri64/RIOT,kbumsik/RIOT,x3ro/RIOT,miri64/RIOT,BytesGalore/RIOT,smlng/RIOT,kbumsik/RIOT,smlng/RIOT,mfrey/RIOT,OlegHahm/RIOT,kYc0o/RIOT,A-Paul/RIOT,rfuentess/RIOT,lazytech-org/RIOT,toonst/RIOT,ant9000/RIOT,authmillenon/RIOT,toonst/RIOT,neiljay/RIOT,OlegHahm/RIOT,basilfx/RIOT,smlng/RIOT,x3ro/RIOT,aeneby/RIOT