Release: update downloads page and release-process docs for 2.0.0

- docs/downloads.md: add Current Release 2.0.0 section with
  dist.apache.org download links, SHA-512, ASC, and KEYS URL
- docs/community/release-process.md: fix Step 4.1 SVN dev path
  (remove /core/ suffix that doesn't exist), update Step 4.2 to
  document actual MkDocs build + rsync workflow replacing the
  old core-staging approach, fix Step 7 to match new site workflow

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>