25.12 - updated Maintainer Docs - Release Planning

[rockhowse]

This PR updates the Release Planning section

• https://docs.rapids.ai/releases/planning/

to bring it up to the current planning/release process post RSN-47

• https://docs.rapids.ai/notices/rsn0047/

This focuses on using CALVER (YY.MM.PP) instead and + an un-changing main development branch

xref: rapidsai/build-planning#224

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[rockhowse]

ok now that we ahve the labels sorted out... I put this up as a draft PR as therei s one key thing I don't know about.

This page includes a link to a centralized release dashboard of some sort... it originally pointed to a specific repository in cudf.

I added one of the seemingly related "release dashboards" that ops used during the release itself created by @AyodeAwe , but I don't think that's correct either.

Will need some input to get this set up and aligned with the current release planning paradim and discuss it with @mmccarty as I believe he has been one of the folks driving the rapids planning.