docs: documentation rework + Vortex encoding-format specification

[mprammer]

Summary

Refreshes the Vortex documentation and adds a specification of the on-disk / on-wire format.

• Documentation structure — a consolidated top-level Specification section, the developer guide grouped by audience, and refreshed concepts / user-guide / getting-started pages, with redirects for moved pages.
• Encoding-format specification — the physical byte layout of the stable encodings: the cross-cutting validity contract, a per-encoding byte-layout reference for every stable encoding, and the array / scalar / file / dtype framing, plus a "reading a file" walkthrough.
• Docs conformance harness — scripts/encoding_stability.py derives the stable-encoding set from the code, and scripts/check-docs-conformance.py checks the docs stay consistent with it; both are wired into CI as a guardrail against drift.

The warning-strict docs build is clean and the conformance checks pass.

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will improve performance by 12.39%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚡ 2 improved benchmarks
✅ 1643 untouched benchmarks
⏩ 4 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	chunked_varbinview_into_canonical[(100, 100)]	307.5 µs	272.5 µs	+12.84%
⚡	Simulation	bitwise_not_vortex_buffer_mut[128]	273.6 ns	244.4 ns	+11.93%

Tip

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_{Comparing mp/spec (42cce13) with develop (4c762f7)}

Footnotes

1. 4 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩