Add DocFX documentation system and slim README

[DJGosnell]

Summary

Set up a DocFX-based documentation system for Logsmith, mirroring the NexNet pattern (Dtronix/NexNet#72). Extracts all detailed content from the ~1400-line README into 12 focused doc articles and 2 internals pages, adds a landing page with feature grid, API reference config, and a GitHub Actions workflow for GitHub Pages deployment.

Reason for Change

The README contained all documentation inline — installation, modes, sinks, formatting, structured output, scoped context, sampling, dynamic levels, MEL bridge, abstraction mode, testing, configuration, architecture, and diagnostics. This made it difficult to navigate and maintain. Moving to a proper documentation site with DocFX provides searchable, structured documentation with auto-generated API reference.

Impact

• README: Slimmed from ~1400 to ~110 lines. Now contains: logo, CI badge, tagline, package table, "Why Logsmith" pitch, comparison table (vs MEL+LoggerMessage/ZLogger/Serilog/NLog), feature list with doc links, quick start sample, benchmarks link, and license.
• Documentation site: 12 articles covering all Logsmith features, landing page with feature grid, separate Internals section for architecture and compile-time diagnostics, and auto-generated API reference for Logsmith and Logsmith.Extensions.Logging.
• CI/CD: New docs.yml workflow builds DocFX (pinned v2.78.5) and deploys to GitHub Pages on push to master.

Plan items implemented as specified

• Phase 1: DocFX infrastructure (docfx.json, filterConfig.yml, toc files, custom template, .gitignore)
• Phase 2: Landing page with feature grid (Zero-Allocation Pipeline, Source-Generated, Three Modes, Structured JSON Output, Compile-Time Level Stripping, MEL Bridge)
• Phase 3: Core articles (Getting Started, Declaring Log Methods, Log Levels, Sinks)
• Phase 4: Middle articles (Formatting & Output, Scoped Context, Advanced Features, MEL Bridge)
• Phase 5: Remaining articles (Operating Modes, Performance, Testing, Configuration Reference)
• Phase 6: Internals pages (Architecture, Diagnostics)
• Phase 7: README rewrite to ~110 lines
• Phase 8: GitHub Actions docs deployment workflow
• Phase 9: Final verification (all 266 tests green, all links verified)

Deviations from plan implemented

• README slimmed to ~110 lines instead of planned ~150-200 (more aggressive than planned, all content preserved in docs).
• Exception Handling and Explicit Sink Parameter content placed in performance.md instead of configuration-reference.md and testing.md (better thematic fit).

Gaps in original plan implemented

• "What Logsmith Is Not" section (MEL ecosystem caveats, no runtime template engine) was initially missed — added to getting-started.md during review remediation.
• Benchmarks link in README was initially a relative path — fixed to doc site URL during review remediation.
• DocFX tool version was initially unpinned — pinned to v2.78.5 during review remediation.

Migration Steps

• GitHub Pages must be enabled on the repository (Settings → Pages → Source: GitHub Actions) for the docs workflow to deploy.

Performance Considerations

No runtime impact — documentation only.

Security Considerations

• docs.yml permissions are scoped to contents: read, pages: write, id-token: write.
• DocFX pinned to v2.78.5 for supply-chain safety.
• No secrets or sensitive data in documentation.

Breaking Changes

• Consumer-facing: README structure changed significantly. Direct links to README anchors (e.g., #sinks, #log-levels-and-conditional-compilation) will break — content now in doc articles.
• Internal: None.