docs(readme): add workbench plug above the fold

[fede-kamel]

Summary

Surface the Workbench prominently at the top of the README. Before this PR, the only mention was a single word in the centered nav row, linking to the in-repo workbench/ source dir. After: a dedicated CTA right below the nav, with a one-line pitch and a direct link to the Workbench guide. Also fixed the existing nav Workbench entry to point at the public docs page instead of workbench/.

Diff

<p align="center">
  <strong>Try every locus pattern in your browser →</strong>
  <a href="https://oracle-samples.github.io/locus/workbench/"><strong>Workbench guide</strong></a><br>
  <em>Step-by-step setup for the browser playground — run it on localhost
  in three terminals, or in a single Docker container. Bring your own
  OpenAI / Anthropic key, or wire up an OCI profile.</em>
</p>

Test plan

• Markdown renders correctly (markdownlint pre-commit pass).
• CI green.