Sort docs by reader intent

[vincebln2]

Fixes #328.

The docs taxonomy already defines tutorials as learning, how-to guides as doing, reference as lookup, and explanation as understanding, but several files still lived in the wrong quadrant or duplicated one guide per language.

What changed

• Merged the JavaScript and Python downloads docs into one how-to guide, and did the same for accessibility trees
• Moved window and viewport guidance into how-to, recording format into reference, and maintainer setup/publishing runbooks into contributing
• Renamed the misspelled classic WebDriver reference while retaining one BiDi explanation and one BiDi protocol reference
• Updated every inbound link and the executable tutorial test paths
• Kept using-firefox.md and record-video.md at their existing paths; the Firefox capability table is unchanged

Verified:

• Relative Markdown link audit passes across the repository, excluding the vendored W3C spec syntax
• JS and Python tutorial parsers discover all annotated tests at their merged paths
• git diff --check passes