chore(docs): v2.0.0 doc consolidation, fix stale examples + ship MIGRATION + multi-source sample

[fxthiry]

Closes the documentation gaps that were not addressed during the multi-source feature work, before tagging v2.0.0 final.

What ships

• Three stale v1.x example configs (README.md Quick Start, docs/getting-started.md minimal, examples/cisco-switches/config.yaml) migrated to the v2.0.0 map shape with a default: source. They would otherwise have been rejected by --validate after a copy-paste.
• MIGRATION.md at repo root: 5-section guide for v1.x upgraders covering the pre-upgrade checklist, config before/after, Prometheus metric migration with PromQL examples, notifier output format changes (Mattermost footer + default webhook payload), and a rollback path. Linked from README.md and CHANGELOG.md.
• examples/multi-source/: complete reference config (2 sources, 3 rules covering pinned single-source + fan-out) plus a README.md explaining the routing matrix. Closes the explicit ask from the issue #34 reporter.
• README Features list reworked: multi-source bullet is now position Welcome to Valerter ! #1 with an outcome-led wording ("One Valerter for every VictoriaLogs you run"). Blockquote callout right after Quick Start points v1.x upgraders at MIGRATION.md.
• docs/performance.md accuracy banner: throughput numbers in that page were measured against v1.x (single task per rule); the v2.0.0 N×M task model and max_streams cap are now mentioned at the top with a note that re-measurement is pending.
• Two new integration tests lock the shipped samples against future drift:
  • shipped_config_example_validates runs --validate on config/config.example.yaml.
  • shipped_examples_pass_validate iterates examples/*/config.yaml and runs --validate on each.

Validation

• cargo test — 551 tests pass (479 lib + 72 integration, +2 new in integration_validate)
• cargo clippy --all-targets --all-features -- -D warnings — clean
• cargo fmt --check — clean
• All three shipped example files: valerter --validate -c <path> → exit 0
• Manual: README Quick Start YAML extracted to a temp file, --validate → exit 0
• Manual link check: every internal markdown link added (README → MIGRATION, README → examples/multi-source, MIGRATION → CHANGELOG sections) resolves on disk

Out of scope (deferred to follow-up)

• Tagline reword (e.g. "One alerting control plane for all your VictoriaLogs")
• ASCII / SVG diagram for multi-source routing
• "Concepts" page bridging README and configuration.md
• GHSA filing for the email body_html security advisory
• Snippet harvester test that validates inline ```yaml blocks in .md files
• Runtime error message that points users at MIGRATION.md when the legacy victorialogs.url shape is detected (code change, separate concern)