Rebrand manager contracts to splits

[OisinKyne]

Summary

Rebrands manager contracts to splits.

Details

Important to note that: this line https://github.com/ObolNetwork/obol-docs/blob/main/src/components/HomepageFeatures.tsx#L50

will need to be updated with the below snippet when we cut the next version, or the build will fail as I've changed a filename.

description: (
    <>
      <Link to="/docs/sc/introducing-obol-splits">Obol Splits</Link> are
      smart contracts for the distribution of rewards from Distributed Validators.
    </>
  ),
  link: "/docs/sc/introducing-obol-splits",

ticket:

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