docs: comprehensive documentation for ExploreDNS

- Rewrite README.md with overview, features, installation (go install +
  build from source), quick start, full CLI flag reference table,
  output section descriptions, project structure, and development guide

- Add package-level doc comments to all five internal packages:
  config, dns, fingerprint, output, traverse (via doc.go or existing
  package-declaration files)

- Add GoDoc comments on every exported type, constant, function, and
  method across all packages:
  - internal/config: Config struct fields, all Parse*/Default/Validate
  - internal/dns: QueryConfig, Resolver, BasicResolver, CachingResolver,
    ExchangeFunc, RootServer, RootDiscoveryConfig, DecodedResponse,
    ResponseClassification, all exported helpers
  - internal/fingerprint: Fingerprinter, New, NewWithTimeout, Query,
    FingerprintAll
  - internal/traverse: Traverser, TraverserConfig, TraversalResult,
    Referral, ResolutionState, Response, ResponseType, InfoCache,
    Stack, TraverserHooks, EventStage, TraversalEvent, EventHandler,
    CircularReferralError, UnresolvableNameserverError
  - internal/output: Format, Config, Formatter, SummaryStats,
    NewFormatter, AttachHooks, RunTraversal, DefaultConfig,
    ComputeSummary

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

internal/dns/dns.go

@@ -1 +1,11 @@
+// Package dns provides low-level DNS query primitives, response decoding, root
+// server discovery, and type constants used throughout ExploreDNS.
+//
+// The package is organised around four concerns:
+//
+//   - types.go     — DNS record-type constants and helpers (QNameType, etc.)
+//   - query.go     — Query / IterativeQuery and retry/back-off logic
+//   - resolver.go  — Resolver interface plus BasicResolver and CachingResolver
+//   - decode.go    — DecodeResponse, ResponseClassification, and related helpers
+//   - roots.go     — DiscoverRoots for bootstrapping iterative traversal
 package dns