Publish v0.1.0-beta.1

[SamDozer]

The v0.1.0-beta.1 release stays a draft until it is signed with a permanent
key. Depends on #1.

• Rebuild artifacts with the release key
• Generate SHA256SUMS.txt
• Attach the arm64-v8a, armeabi-v7a, x86_64 and universal APKs plus the
  checksums
• Upload the social preview image, docs/branding/github-social-preview.png
  — this is a web-UI-only setting, the API cannot do it
• Install from the published artifact rather than a local build, and open
  it once
• Publish as a prerelease

Release notes are drafted in docs/release/v0.1.0-beta.1.md.