Add Plomin hard fork documentation

docs/about-cardano/05-evolution/02-eras-and-phases.mdx

@@ -63,6 +63,13 @@ The Vasil upgrade significantly enhanced Cardano’s performance and developer e
 
 These features collectively improved the efficiency, expressivity, and scalability of the Cardano network, laying a stronger foundation for advanced smart contracts and DApp development.
 
+The Valentine (SECP) upgrade, executed on February 14, 2023, was an intra-era hard fork in the Babbage ledger era that followed Vasil. It introduced new Plutus built-in functions for SECP elliptic curves—enabling native verification of ECDSA and Schnorr signatures on Cardano. This focused change improves interoperability and developer experience without changing the ledger era.
+Key facts:
+- Intra-era hard fork that kept the ledger in Babbage and followed Vasil
+- Added Plutus built-ins for ECDSA and Schnorr; implemented and audited by experts to provide a high level of security
+- Helps DApp developers verify signatures from other ecosystems (including EVM sidechains) more simply and cost-effectively
+- Took place on February 14, 2023 (epoch 394), protocol version 8.0.
+
 ### Conway era
 
 The Conway ledger era is part of Cardano’s _Voltaire development phase_, focused on transforming the network into a self-governing system. It encompasses two hard forks: _Chang_, which introduced the initial governance framework, and _Plomin_, which completed the governance feature set (including DReps, treasury withdrawals, and full governance actions) under [CIP-1694](https://cips.cardano.org/cip/CIP-1694).

docs/about-cardano/05-evolution/04-upgrades/08-plomin.mdx

@@ -0,0 +1,31 @@
+---
+title: Plomin
+metaTitle: Plomin - the second governance upgrade in the Conway era
+---
+
+# Plomin hard fork
+
+Plomin, the second governance upgrade in the Conway era that completes Cardano's on-chain governance features under [`CIP-1694`](https://github.com/cardano-foundation/CIPs/tree/master/CIP-1694) and marks the transition to community-driven decision-making. See also the companion explainer for [Chang](https://docs.cardano.org/about-cardano/evolution/upgrades/chang).
+
+## What is Plomin
+
+Plomin is the January 29, 2025 hard fork (epoch 537, protocol version 10.0) that introduces the **second batch** of CIP-1694 governance capabilities: it enables the full set of governance actions and activates the **delegated representative (DRep)** role for ada holders.
+
+## Why it matters
+
+With Plomin, Cardano completes the Conway governance era and moves to **full decentralized governance**. ADA holders can now participate directly or via DReps in decisions such as protocol updates, treasury withdrawals, and future hard forks. The upgrade was enacted following approval by stake pool operators (SPOs) and the interim constitutional committee (ICC), making it the first community-driven hard fork in Cardano history.
+
+## Background
+
+Cardano introduced on-chain governance in two stages within the Conway era: **Chang #1** and **Chang #2**. In November 2024, the community affirmed renaming the second stage to **Plomin** in honor of Matthew Plomin.
+
+## Governance scope after Plomin
+
+Plomin enables a **tripartite** governance system in which representative bodies can consider and ratify major decisions. In the immediate post-fork period, Cardano’s elected delegates focus on three foundational items: **the Constitution, the budget, and the roadmap**—each to be decided through on-chain processes now made possible by Plomin. 
+
+## Feature overview
+
+* Full set of CIP-1694 governance actions
+* Activation of the DRep role for delegated voting
+* Community-driven hard fork, approved by SPOs and the ICC
+* Completion of the Conway governance era under Voltaire.