Improve docs IA, API reference depth, and PR docs preview workflow

[Copilot]

The docs were functional but shallow: key HttpNegotiateAuth behaviors were undocumented, the Sphinx site was a single page, and contributors had no PR-visible rendered docs preview. This updates the user and developer documentation around the actual auth flow and adds a docs workflow that builds on PRs and uploads the rendered HTML artifact.

• README overhaul

  • tightened the package positioning around Windows SSPI + httpx2
  • added platform constraints, quick start, explicit-credential usage, delegation, SPN override guidance, troubleshooting, and developer entry points
  • reduced ambiguity around default credentials vs explicit credentials

• API documentation

  • added package/module overview docstrings
  • expanded HttpNegotiateAuth documentation to cover:
    • SPN host selection and canonicalization
    • channel binding / Extended Protection behavior
    • retry semantics and cookie propagation
    • existing Authorization header short-circuit
    • NTLM challenge validation and failure modes
  • documented internal helper methods that carry security- and protocol-relevant behavior

• Sphinx restructuring

  • converted the docs from a single-page layout into focused sections:
    • getting started
    • usage guide
    • API reference
    • troubleshooting
    • contributing/development
  • updated index.rst to act as a landing page with a real toctree
  • enabled better API browsing with Sphinx viewcode and explicit version metadata

• Contributor and workflow improvements

  • documented local docs/test/build commands and the auth-flow architecture
  • updated the docs workflow to run on pull requests and pushes to main
  • uploaded rendered HTML as a docs-html artifact so reviewers can inspect doc changes without building locally

Example of the expanded usage guidance:

import getpass
import httpx2
from httpx2_negotiate_sspi import HttpNegotiateAuth

auth = HttpNegotiateAuth(
    "svc-http-client",
    getpass.getpass(),
    "CONTOSO",
)

with httpx2.Client(auth=auth) as client:
    response = client.get("https://iis.contoso.com")
    response.raise_for_status()