Add a link to the libp2p community / public events calendar

[BigLep]

Done Criteria

https://libp2p.io/#communitymakes it clear that:

1. We have a community calendar
2. What it's purpose is (surface events for libp2p implementers and consumers)
3. How to view it
4. How to add items to the calendar

Notes

1. The libp2p Public Events calendar is visible here: https://calendar.google.com/calendar/embed?src=libp2p.io_0q9682i3te7eanhe9q7ae1c58g%40group.calendar.google.com
2. Events can be added to the libp2pcalendar by inviting libp2p.io_0q9682i3te7eanhe9q7ae1c58g@group.calendar.google.com

[BigLep]

@p-shahi: this could be a good quick win to knock out. Feel free to put me on the PR.

[BigLep]

Bonus items:

1. Link to our community stuff from https://github.com/libp2p/libp2p (I think we should have one source of truth but link from various places)
2. Do a discuss.libp2p.io post so folks are aware

Related: there is a similar request in IPFS: ipfs/ipfs-docs#1237

[BigLep]

If modifying https://libp2p.io/#communitymakes requires webdev skills, an alternative would be to add a button for "Calendar" that links to a section in https://github.com/libp2p/libp2p which can have more details about how to view and add to the calendar.