updates Readme to link to active repos & minor style fixes

[m4sterbunny]

This PR implements issue(s) #1868

Checklist

• [ na] I wrote new tests for my new core changes.
• [ na] I have successfully ran tests, style checker and build against my new changes locally.
• [ na] I have informed the team of any breaking changes if there are any.

---

Note

Updates README with client compatibility details, refreshed repository links, and minor wording/style tweaks.

• Docs (README):
  • Add execution/consensus client compatibility details: Besu/Geth with Maru.
  • Refresh repositories list:
    • Clarify this repo (linea-monorepo) and note it maintains Linea-Besu plugins for sequencer/RPC nodes.
    • Replace linea-besu with linea-besu-upstream; remove linea-sequencer.
  • Minor copy/style edits: heading capitalization, phrasing of local dev guide, security policy casing, and note that docs are managed from a public repo.

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[eloi010]

Now that we're using Besu, I'd just say "Besu" instead of "linea-besu" here:

- [Besu](https://github.com/hyperledger/besu/): Besu execution client

Maybe also list and link the mentioned plugins.

Finally, I'd also consider adding:

• Linea Besu Upstream: Upstream Besu build for Linea
• Maru: Consensus client used by Linea L2.

@julien-marchand what are your thoughts on this? anything missing here?

[m4sterbunny]

@eloi010 Julien had similar comment on the constraints update --> my proposal is line 27: add besu to the "what is" section. Julien approved this for constraints, does it work here too?

[eloi010]

@julien-marchand is the best one to confirm here. I'd still consider mentioning other repos like Linea Besu Upstream. For Maru in line 27, I'd explicitly say that it is the consensus client in use.

[m4sterbunny]

@eloi010 apologies, you did clearly did mention that repo: added with suggested summary line