make readme current & minor style fixes

[m4sterbunny]

Rmvs links to archived repos to keep readme current
minor style fix and better linking for docs

---

Note

Updates README to reflect current repos, add Besu/Geth + Maru node compatibility, and make small style/link adjustments.

• README (README.md):
  • Ecosystem repos:
    • Remove archived entries for linea-besu and linea-sequencer.
    • Clarify linea-monorepo as the main stack repo and note it maintains Linea-Besu plugins.
  • Node compatibility: Add note that Linea works with Besu or Geth, paired with Maru for full nodes.
  • Contributing: Normalize casing in "Guidelines for non-code and other trivial contributions".
  • Useful links: Update "Linea docs" to mention it’s managed from a public repo with link.

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