docs: add Get Started section to README

[LorrisSaintGenez]

What

• Add a "Get started" section to the README: sign in (algolia auth login), choose an application (list / select / create), and a first index + search micro-action.
• Add a short note in CONTRIBUTING.md on how to run the locally built binary (./algolia your_command ...).

GROUT-327

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

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