docs(readme): document StarPU backend, deprecate PaRSEC, add local benchmark results

[jfonseca-aneo]

Summary

Updates the README to reflect the current state of the distributed backends after the switch to StarPU in CI:

• Documents that the task-based parallelization can run on either StarPU (now CI's default distributed backend) or PaRSEC, and marks PaRSEC as deprecated (kept behind -DSEWAS_WITH_PARSEC=ON, no longer built/tested by CI).
• Adds a deprecation note next to SEWAS_WITH_PARSEC in the build options section, pointing users to SEWAS_WITH_STARPU going forward.
• Adds a new "TestA results with the StarPU backend" section with local single-process benchmark output (Intel Core i7-1255U, 10c/12t, 30 GiB RAM, Linux) for comparison against the existing PaRSEC numbers.

Why

Follow-up to the CI backend switch (fb316a3) and the new SEWASStarPU execution engine, so the README no longer describes PaRSEC as the only/primary runtime.

Test plan

• Docs-only change, no code affected.