Fix: broken/redirected links in developers docs

[MOSHKA-GOT]

Description

This PR fixes broken or redirected links and performs minor text hygiene updates across public/content/developers/docs/*:

• Proof of Authority (PoA)

  • Updated IBFT 2.0 link to the current Hyperledger Besu documentation.
  • Re-formatted the implementation list (Clique / Aura / IBFT 2.0) without changing content.
  • Kept reference to EIP-225 as the canonical spec.

• Proof of Stake → Weak Subjectivity

  • Updated Teku docs link (removed utm_source, pointed to the active canonical URL).

• Frameworks

  • Replaced the deprecated OpenZeppelin SDK link with OpenZeppelin Defender SDK documentation.

• MEV → Builder API

  • Reordered and cleaned the Flashbots MEV-Boost references — now points primarily to the official Flashbots Docs overview, GitHub link kept as secondary.

• Nodes & Clients → Run a Node

  • Fixed Stereum link (project moved from stereum.net/... to stereum-dev.github.io/...).

• Smart Contracts → Formal Verification

  • Updated Act documentation link from ethereum.github.io/act to the maintained github.com/argotorg/act.

• Smart Contracts → Testing

  • Replaced the broken “Static analysis of smart contracts with Slither” link (crytic/building-secure-contracts/.../slither#slither) with the official repository github.com/crytic/slither.

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