Clean up documentation drift

[wilsonfreitas]

Summary

• align README, Sphinx docs, examples, and public docstrings with current async, date-input, regional-mode, and TaxaJuros behavior
• document local docs build command and docs dependency group
• update async examples to close the shared async client and show current multi-symbol currency support
• clean minor stale wording and example lint drift

Fixes #36

Verification

• uv run pytest -m "not integration" (185 passed, 11 deselected)
• uv run ruff check bcb/ tests/
• uv run ruff format --check bcb/ tests/
• uv run mypy bcb/
• uv run --group docs sphinx-build -b html docs docs/_build/html
• uv run ruff check bcb/ tests/ examples/async_usage.py examples/currency_exchange.py examples/odata_query.py examples/sgs_time_series.py
• uv run ruff format --check bcb/ tests/ examples/async_usage.py examples/currency_exchange.py examples/odata_query.py examples/sgs_time_series.py