Rebuild the website with Docusaurus

[tisonkun]

Replace the outdated Pelican website with a compact Docusaurus site in the existing site/ directory. The new pages cover the current v3 application, tally tools, community, and release status, link to documentation maintained alongside the code, and remove the outdated member list. Common legacy .html URLs redirect to the new pages.

GitHub Actions validates website changes, publishes preview/<name> branches to ASF staging, and publishes trunk to asf-site. Generated branches retain their history, include the required .asf.yaml, and are cleaned up when a preview PR closes. This replaces the Pelican publishing configuration.

Project metadata stays in .asf.yaml; the build generates /doap.rdf for consumers that require XML. After the website is published to production, a separate change in apache/comdev-projects/data/projects.xml must repoint STeVe from the SVN DOAP to https://steve.apache.org/doap.rdf. Until then, the legacy project feed continues to read SVN.

Live preview: https://steve-website.staged.apache.org/

Validation:

• pnpm validate: formatting, metadata/serialization tests, production build, and broken-link checks.
• Built-output smoke checks: five pages, logo, DOAP, eight legacy redirects and their targets, and generated ASF configuration.
• actionlint for both website workflows.
• uv run ruff format ., uv run ruff check --fix ., and uv run mypy --ignore-missing-imports . in v3/.

[tisonkun]

Some links to the GitHub repo, especially to the trunk branch, may be unstable. We can improve the links and information framework later.

Merging for now.