docs: clarify Python SDK quickstart

[daniel-scrivner]

Scope

• Make the README start with installation, a contract-valid filing request, expected result, docs, compatibility, and support.
• State the SDK's REST wrapper posture, hosted MCP boundary, and x-api-key authentication.

Factual self-audit

• SecApiClient, agent_latest_filing(), SECAPI_API_KEY, and x-api-key were verified against this branch and current canonical docs.
• The example records accession number, filing date, filing URL, and request ID; it does not promise a stable live filing.
• No API behavior, package version, or package manifest changed.

GitHub metadata

• Applied description: Python SDK for source-backed SEC filings and financial data.
• Homepage remains https://docs.secapi.ai; added sec-filings topic.

Registry delta

• No PyPI publish attempted. Observed PyPI secapi-client==1.1.0 summary: Python SDK for SEC API, with Python >=3.11, docs URL https://docs.secapi.ai, and homepage https://secapi.ai/developers.
• The source version and project URLs remain unchanged; the published project description will remain at the prior release until the normal PyPI release owner publishes a new version.

Validation

• python3 -m unittest discover -s tests -v (all pass)
• git diff --check
• Documentation-link HTTP checks

Tracks OMNI-5503 under OMNI-5326.