docs: cover beta releases and renamed server setting

[sivert-io]

What changed

• document latest-beta, exact beta tags, rollback precautions, and desktop prerelease installers
• replace GRYT_AUTH_MODE with GRYT_SERVER_ENABLED across the four affected docs pages
• explain that GRYT_IDENTITY_TIERS, not the maintenance switch, controls whether accounts are required
• document accepted boolean values, empty-value behavior, invalid values, and compatibility with the old variable

Checks

• next build --webpack
• mechanical Humanizer and AI-writing-tell scan of the added prose
• git diff --check

The default Turbopack build cannot follow the isolated worktree dependency symlink, so the production build was run with the webpack builder.

Tasks: GRYT-279, GRYT-281

Related changes:

• Rename GRYT_AUTH_MODE to what it actually does (GRYT-281) server#49
• fix: pass GRYT_SERVER_ENABLED through Compose gryt#82
• fix: configure embedded server advertised addresses client#156