Refresh documentation for D2 v0.9.0

[alixander]

Human

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AI

Several pages still describe older D2 behavior, and the release overview points to v0.7.1. Update the overview to v0.9.0, add the published v0.8.2 and v0.9.0 release notes, and refresh the tour's TALA, export, layout, font, and JavaScript guidance.

Document ELK's alternative algorithms, TALA's grid-cell routing, and the requirement to set top and left together, with a runnable positioning example. Refresh the CLI manual from v0.9.0, remove obsolete Markdown SVG troubleshooting, and update the homepage and roadmap to reflect shipped features and export formats.

Validation:

• yarn run prod passes with the existing SVG/HTML minifier warnings.
• The documented ELK force, stress, and radial commands and the new TALA positioning example compile successfully.
• Updated pages checked in the local browser; git diff --check passes.