docs: rewrite product documentation

[vavkamil]

Description

This PR rewrites and polishes the CVEalert documentation for the MVP release.

• Renames the main settings documentation from Projects/Webhooks/User to Organization/Integrations/Account.
• Reworks the Getting Started, Application, Software, and Settings guides around the current app navigation.
• Adds concise onboarding, triage, monitoring, role, account security, integration, and FAQ guidance for technical users.
• Adds and reorders application preview screenshots so the docs follow the expected product flow.
• Uses Zensical features such as admonitions, cards, tabs, custom icons, and small stylesheet polish for a cleaner reading experience.
• Applies a final editorial pass for consistency, tone, terminology, and readability.

Notes

• The app frontend is treated as the source of truth for names, routes, and visible workflows.
• Old settings documentation slugs are removed as part of the MVP rename; compatibility redirects are not included in this PR.
• Slack and Telegram are documented as the currently configurable integrations. Email is coming soon; Custom API, RSS, CSV Export, and MS Teams are planned; Discord is optional.
• Local validation passed with uv run zensical build --clean and npx markdownlint-cli2 --config .github/.markdownlint-cli2.yaml "docs/**/*.md" "!site/**".

Checklist

• I have updated the PR description and explained my changes
• I have updated the README/documentation, if necessary
• I invested in the quality of the code I wrote
• CI builds are valid and all tests are passing