chore: redirect the legacy GitHub Pages URL to extenddb.org

[yesyayen]

What

Adds a root index.html that redirects the legacy GitHub Pages URL (extenddb.github.io/extenddb) to https://extenddb.org/.

Why

https://extenddb.org/ is the only authoritative source for ExtendDB documentation. The legacy Pages site is being deleted and archived; this redirect points any remaining visitors and bookmarks to extenddb.org in the interim. An admin can then disable Pages in the repo settings (Settings -> Pages -> Source: None).

Closes n/a

Testing done

Verified the page locally in a browser: it redirects immediately to https://extenddb.org/. noindex keeps the legacy URL out of search results.

Checklist

• I have read CONTRIBUTING.md
• All tests pass (cargo test --workspace) - no Rust changes
• Code is formatted (cargo fmt --check) - no Rust changes
• Clippy is clean (cargo clippy -- -W clippy::pedantic) - no Rust changes
• I have added or updated tests for new functionality - n/a
• I have updated documentation if behavior changed - n/a
• Breaking changes are noted below (if any) - none

ADR / RFC: n/a

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the terms of the Apache License 2.0 and I agree to the Developer Certificate of
Origin (DCO). See CONTRIBUTING.md for details.