Readded talks and slides page in wiki

Hi I readded

from the old wiki :
which I think is a useful overview of OpenWrt and people involved.

Currently it is not linked anywhere from the main page where it was linked in before, because I do not have admin access.

I noticed some issues with older content (prplfoundation site) that was removed and I replaced that with link.

Having a backup of all or a part of that OpenWrt history archived might also be a good thing to have - it doesnt have to be publicly accessible because of copyright and redistribution issues. (or raw pre-uploaded footage)

I didnt split associated generic links or events like Freifunk or other potential links (feel free to add them/modify) because while they use forks / custom OpenWrt builds they might be just as valid as commercial entities using their own OpenWrt forks and presenting them at embedded conferences or OpenWrt Summit.