new IRC channel

As agreed in a Technical Board meeting, the preferred IRC channel for the DPDK project moved from freenode to Libera.Chat:

	https://mails.dpdk.org/archives/dev/2021-July/214662.html

The website is updated:
https://core.dpdk.org/contribute/

There is a good guide to start with Libera.Chat:
https://libera.chat/guides/connect

Let's meet on Libera.Chat channel #DPDK
for quick questions, synchronization, or just to say hello!