docs: release notes (#3794)

Closes #3791

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Co-authored-by: Sanaz Taheri <[email protected]>
Co-authored-by: Callum Waters <[email protected]>

docs/release-notes/pending-release.md

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+# Pending Nelease
+
+## v3.0.0 [Unreleased]
+
+### Node Operators
+
+- Consensus node operators should enable the BBR (Bottleneck Bandwidth and Round-trip propagation time) congestion control algorithm. See [#3774](https://github.com/celestiaorg/celestia-app/pull/3774).
+
+### Library Consumers
+
+- Namespace and share constants in the `appconsts` package were moved to [celestiaorg/go-square](https://github.com/celestiaorg/go-square). See [#3765](https://github.com/celestiaorg/celestia-app/pull/3765).

docs/release-notes/release-notes.md

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+# Release Notes
+
+This guide provides notes for major version releases. These notes may be helpful for users when upgrading from previous major versions.
+
+## [v2.0.0](https://github.com/celestiaorg/celestia-app/releases/tag/v2.0.0)
+
+### Node Operators
+
+If you are a consensus node operator, please follow the communication channels listed under [network upgrades](https://docs.celestia.org/nodes/participate#network-upgrades) to learn when this release is recommended for each network (e.g. Mocha, Mainnet Beta).
+
+Consensus node operators are expected to upgrade to this release _prior_ to the Lemongrass hardfork if they intend to continue participating in the network. The command used to start the [consensus node](https://docs.celestia.org/nodes/consensus-node#start-the-consensus-node) or [validator node](https://docs.celestia.org/nodes/validator-node#run-the-validator-node) will accept an additional `--v2-upgrade-height` flag. See [this table](https://docs.celestia.org/nodes/hardfork-process#lemongrass-hardfork) for upgrade heights for each network.
+
+Consensus node operators should enable the BBR (Bottleneck Bandwidth and Round-trip propagation time) congestion control algorithm. See [#3812](https://github.com/celestiaorg/celestia-app/pull/3812).
+
+### Library Consumers
+
+If you are a library consumer, a number of the Go APIs have changed since celestia-app v1.x.x. Some of the notable changes are:
+
+- Code pertaining to the original data square was extracted to [celestiaorg/go-square](https://github.com/celestiaorg/go-square).
+  - celestia-app v1.x had a shares package. celestia-app v2.x uses [go-square/shares](https://github.com/celestiaorg/go-square/tree/c8242f96a844956f8d1c60e5511104deed8bc361/shares)
+  - celestia-app v1.x had a blob.types package with `CreateCommitment` function. celestia-app v2.x uses `CreateCommitment` function from the [go-square/inclusion](https://github.com/celestiaorg/go-square/tree/c8242f96a844956f8d1c60e5511104deed8bc361/inclusion).
+- celestia-app v1.x had a lot of functionality included in the signer. celestia-app v2.x splits a txClient from the signer. See [#3433](https://github.com/celestiaorg/celestia-app/pull/3433).