Update "announcements" with new pymcon link

[reshamas]

We should update the announcement cards with new pymcon website: https://www.pymc.io/welcome.html#announcements.

The announcements in the landing page need to be updated too

Originally posted by @OriolAbril in #58 (comment)

This is continuation work from #58

[OriolAbril]

what announcements do we want to have in the landing page now?

some options:

• 3x2 layout, keeping all current announcements. That would be adding pymcon announcement as the first one
• 2 rows, 1 element in the first and 3 in the 2nd. We could for example have pymcon be a larger and wider announcement, then have 3 older/smaller announcements in the 2nd row: pytensor, v4 and office hours?
• keep proposing