Add link to release notes for subsequent v2.x.x releases

[rootulp]

Context

Slack thread

Problem

Luka asked if we could add hardfork info into the release notes for subsequent v2.x.x releases.

Proposal

1. [From @cmwaters] We could add a link to the upgrade doc by default for all releases
2. [From @rootulp ] If we can't do ^ then we can add a line to the https://github.com/celestiaorg/celestia-app/blob/main/docs/maintainers/release-guide.md for maintainers to manually update the release notes with a link to the https://github.com/celestiaorg/celestia-app/blob/main/docs/release-notes/release-notes.md

[rootulp]

I did some housekeeping and attached a link to release notes on recent v2.x.x releases. I think we could add a link to release notes via GoReleaser created releases but most of our releases are created via Github UI new release page so that wouldn't work. Apparently Github doesn't have a template for those release notes so will add it to our release guide.