docs: rewrite README with professional layout and branding

[deepak7340]

Restructure the README around how LinuxAid is actually used, rather than around its Puppet internals.

Layout

• Centred hero with the linuxaid.io wordmark, tagline, badges, and nav
• Logo vendored as two SVGs: the site version uses currentColor, which resolves to black standalone and would be invisible on GitHub dark mode, so the site's own ink colours are baked in (#11151c light, #e9edf5 dark) with the #eb4926 accent kept in both, served via prefers-color-scheme
• Feature bullets replaced by a capability matrix (configuration, monitoring, security & compliance, automation, platforms)
• Documentation grouped into Setup & Operations, Monitoring & Security, and Reference

How It Works

• Leads with the 3-step add-a-server flow, then describes the agent pipeline: infrastructure -> LinuxAid-Agent -> Prometheus -> Grafana and Alertmanager -> Slack/Mattermost
• Covers what the agent does per server, plus service windows, configuration management, security & compliance, and logging
• OpenVox, Hiera, ENC, and the module layers moved into a collapsed 'Under the hood' section so the detail is kept but not front-loaded

Fixes

• docs/setup/linuxaid_config.md and docs/setup/enc.md were linked but do not exist; point to docs/setup/git_setup.md and puppet_enc.rb instead
• Link guides that existed but were unreferenced: monitoring, updates, release, turris-install, generate-docs, facts
• Add Contributing and License sections