Document Google and Microsoft company sign-in and recovery

[Kevinjohn]

The company-login guides now match the Google and Microsoft implementation merged in #1240 and #1241. Operators can configure the named providers, connect existing accounts, require company sign-in, and restore password mode using the correct matching profile.

• Document first-owner setup, invitations, explicit linking, Microsoft mailbox proof, SMTP, consent, session and offboarding limits.
• Preserve guarded preflight and repair procedures; remove the retired readiness-panel instructions and unused screenshots.
• Update the environment example, configuration/security references, user stories, development guidance and Unreleased changelog. Include new actual sign-in screenshots and regenerate the standalone documentation.

Part of #1219 and #1214. Merge predecessor: #1241 (merged). Microsoft live-tenant validation remains pending in #1219; these guides do not claim it has passed.

Validation: Node 24.19.0; documentation build and navigation checks passed; 15 focused runbook/Compose-documentation tests passed; formatting and diff checks passed. Reviewed rendered company-login, configuration and incident pages, image loading and new screenshots. No application behavior changed and no additional local E2E run was needed. The only test-code change updates the documented bootstrap setting name.

The implementation merge c100c16 passed all seven automatic main workflows, including Chromium, Firefox and WebKit. This documentation PR remains separate and open for review.