docs: consolidate doc trees into mkdocs site, remove stale sdk/docs

[zeevdr]

Summary

• The SDK shipped two divergent doc trees — the published mkdocs site (docs/) and a pdoc-era tree (sdk/docs/) — that had drifted out of sync, including a set_many() example that passed a dict instead of a list of FieldUpdate objects. Both README.mds linked to the stale tree, so users following the published links could land on outdated, incorrect content.
• Picking a single canonical tree (mkdocs docs/) removes the drift risk going forward and ensures the docs users see from the README and the API reference stay in sync.

What changed

• Migrated the async usage guide into docs/guide/async.md, fixing the stale set_many() dict example to use FieldUpdate (verified against the actual set_many signature in sdk/src/opendecree/client.py/async_client.py) and updating cross-links to match the mkdocs nav. The configuration/error-handling content in sdk/docs/ was not migrated as-is — it was superseded by the more accurate docs/guide/connect.md, which already documents check_version, RESOURCE_EXHAUSTED retries, and TimeoutError (things the old sdk/docs/configuration.md was missing).
• Added the new guide to mkdocs.yml's nav and cross-linked it from docs/index.md and docs/guide/watch.md.
• Removed sdk/docs/ entirely, including the pdoc wiring that generated its stale sdk/docs/api/*.html (the Makefile docs target, the pdoc dependency in build/Dockerfile.tools and sdk/pyproject.toml, and the matching .gitignore entry). The Makefile docs target now builds the mkdocs site directly, matching .github/workflows/docs.yml.
• Repointed README.md and sdk/README.md documentation links at the published docs site (opendecree.github.io/decree-python) instead of the removed sdk/docs/ tree, and updated CLAUDE.md's tooling table (pdoc → mkdocs).

Test plan

• make lint — ruff check + format check pass
• make typecheck — mypy passes (29 source files, no issues)
• make test — 298 passed, 13 skipped, 100% coverage
• Built the mkdocs site locally with mkdocs build --strict — succeeds with no broken-link warnings, and site/guide/async/ renders correctly alongside connect/ and watch/
• Repo-wide grep for sdk/docs and pdoc confirms no remaining references

Closes #135

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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