docs: split the reference into Guides and API, and fix the Windows setup

[alesanfra]

Description

• Guides and API. reference.md is split into guides.md (frames by number, approximate frames, threads, hardware decoding, frames on the GPU) and api.md (signatures, arguments, errors, FrameReader, Frame, Batch, constants). The text is moved unchanged; only the links between the pages differ.
• Redirect. scripts/docs_redirects.py, a MkDocs hook, writes reference/index.html, which sends each old anchor to the page that now holds it (for example #hardware-decoding to guides/, #errors to api/). The README on PyPI for 0.6.0 and earlier links to reference/ five times. mkdocs-redirects would keep the anchor but send every one to a single page, so it is not used.
• Quick start. Adds frames in any order and frames=[-1], device="auto", reading videos in parallel with threads, and a note that errors are raised by the first next().
• Development. The Windows setup is now three steps. It says that pacman is MSYS2's package manager and puts uv's tool directory on PATH for meson and ninja, as CI does. It renames MSYS2's link.exe instead of deleting it. The layout table lists build.rs, src/dlpack.rs, and web/.
• Buffer protocol. A new guide, "Frames without a copy", explains what Frame exposes through the buffer protocol and what that saves: no copy per frame, C-contiguous arrays, batches in one block, and writable memory that the decoder never touches again.
• The README and AGENTS.md point to the new pages, and tests/test_read.py names guides.md where it cited reference.md.

The change of license moved to its own pull request.

The new quick start examples were run against the test video. mkdocs build --strict passes, and the redirect script was checked for guide anchors, API anchors, and no anchor. The Windows steps have not been run on Windows.

Checklist

• cargo fmt and cargo clippy --all-targets -- -D warnings pass (no Rust changes)
• uv run --no-sync pre-commit run -a passes
• Tests added or updated, and uv run --no-sync pytest passes (docs only)
• Docs and CHANGELOG.md updated (the changelog is written by release-please)

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