Add release schedule on the releases page

[jbonofre]

The purpose is to give visibility to the community about our release schedule.

This is a first proposal, happy to discuss and refine ;)

@rdblue @danielcweeks @bitsondatadev wdyt ?

[bitsondatadev]

Hey @jbonofre, would you mind moving this over to https://github.com/apache/iceberg/pulls please?

[jbonofre]

@bitsondatadev sure thing ! I will. Thanks.