docs: restructure documentation into focused topic files

[lvlcn-t]

Motivation

The monolithic 781-line README made it hard to find specific
information and was painful to maintain. This PR splits it into
focused, single-topic documentation files under docs/.

Relates to #391 (review)

Tip

This is a large diff — reviewing commit-by-commit is recommended
for a bit of atomicity. Sorry it's so big!

Changes

Commit 1: restructure documentation

• Slim README from 781 → 94 lines (landing page with links)
• Add focused docs: installation.md, configuration.md,
  checks.md, api.md, observability.md
• Add per-check docs under docs/checks/
• Move auto-generated CLI docs to docs/reference/
• Consolidate dev guides under docs/dev/
• Update scripts/gen-docs/gen-docs.go output path
• Update chart/values.yaml + chart/README.md URL references
• Add .markdownlint-cli2.yaml and AGENTS.md

Commit 2: fix markdown lint

• Reformat CHANGELOG.md, CODE_OF_CONDUCT.md, CONTRIBUTING.md
  for lint compliance (80-char wrapping, dash bullets, title case)
• Reformat docs/dev/README.md and
  docs/ownership-metadata-design.md
• Re-add issue Feature: A sparrow should expose its ownership metadata #354 backlink to ownership-metadata-design.md

New docs structure

docs/
  README.md                → documentation index
  installation.md          → binary, container, Helm install
  configuration.md         → startup config, loader, target manager
  checks.md                → checks overview
  checks/{health,latency,dns,traceroute}.md
  api.md                   → HTTP API endpoints
  observability.md         → Prometheus, traces, Grafana
  dev/
    README.md              → developer guide
    traceroute-testing.md  → Kathara test lab guide
    sbom.md                → SBOM generation guide
  reference/               → auto-generated CLI docs

Tests done

• Markdown lint passes (npx markdownlint-cli2 "**/*.md")
• Unit tests succeeded
• E2E tests succeeded

TODO

• I've assigned this PR to myself
• I've labeled this PR correctly

[niklastreml]

Why is this a stacked pr into fix/linting-tests? this can just be merged into main no?

❯ git diff origin/fix/linting-tests --summary
 create mode 100644 .markdownlint-cli2.yaml
 create mode 100644 AGENTS.md
 create mode 100644 docs/README.md
 create mode 100644 docs/api.md
 create mode 100644 docs/checks.md
 create mode 100644 docs/checks/dns.md
 create mode 100644 docs/checks/health.md
 create mode 100644 docs/checks/latency.md
 create mode 100644 docs/checks/traceroute.md
 create mode 100644 docs/configuration.md
 delete mode 100644 docs/dev.md
 create mode 100644 docs/dev/README.md
 create mode 100644 docs/dev/traceroute-testing.md
 create mode 100644 docs/installation.md
 create mode 100644 docs/observability.md
 rename docs/{ => reference}/sparrow.md (100%)
 rename docs/{ => reference}/sparrow_completion.md (100%)
 rename docs/{ => reference}/sparrow_completion_bash.md (100%)
 rename docs/{ => reference}/sparrow_completion_fish.md (100%)
 rename docs/{ => reference}/sparrow_completion_powershell.md (100%)
 rename docs/{ => reference}/sparrow_completion_zsh.md (100%)
 rename docs/{ => reference}/sparrow_run.md (100%)

sparrow on  refactor/docs [✘] via  v1.26.1
❯ git diff main --summary
 create mode 100644 .markdownlint-cli2.yaml
 create mode 100644 AGENTS.md
 create mode 100644 docs/README.md
 create mode 100644 docs/api.md
 create mode 100644 docs/checks.md
 create mode 100644 docs/checks/dns.md
 create mode 100644 docs/checks/health.md
 create mode 100644 docs/checks/latency.md
 create mode 100644 docs/checks/traceroute.md
 create mode 100644 docs/configuration.md
 delete mode 100644 docs/dev.md
 create mode 100644 docs/dev/README.md
 create mode 100644 docs/dev/traceroute-testing.md
 create mode 100644 docs/installation.md
 create mode 100644 docs/observability.md
 rename docs/{ => reference}/sparrow.md (100%)
 rename docs/{ => reference}/sparrow_completion.md (100%)
 rename docs/{ => reference}/sparrow_completion_bash.md (100%)
 rename docs/{ => reference}/sparrow_completion_fish.md (100%)
 rename docs/{ => reference}/sparrow_completion_powershell.md (100%)
 rename docs/{ => reference}/sparrow_completion_zsh.md (100%)
 rename docs/{ => reference}/sparrow_run.md (100%)