[cooker] Wiki pages for 4.0 RC

Tags: #<Tag:0x00007fe4a12dce58> #<Tag:0x00007fe4a12dcd68>

I have created wiki pages for 4.0 RC

https://wiki.openmandriva.org/en/4.0/RC
https://wiki.openmandriva.org/en/4.0/RC/Release_Notes
https://wiki.openmandriva.org/en/4.0/RC/Errata

In view and with regard to RC release, from now on please edit the RC pages.

As a remind, the 4.0/New page is meant to summarize OMLx 4.0 features
instead, and it belongs to entire OMLx 4.0 release cycle.

Developers are welcome to check and review them as needed.