M2 — Link orphaned Scout docs & rebuild docs/README.md as a complete index

Sub-issue of #37124. Scout half of the Rock — mirrors Jose's M2 (#37127).

Description

Make every doc in Fabrizzio's slice reachable, and rebuild docs/README.md into a real index. Jose's M2 (#37127) linked the backend orphans into CLAUDE.md's navigation; this is the other half — the remaining orphans plus the index that was supposed to catch them.

docs/README.md is not a docs index today: it carries 11 links and 10 of them are frontend. It reads as an index of docs/frontend/, so this is close to writing it from scratch for the other 40+ files.

Acceptance Criteria

• docs/testing/PERFORMANCE_TESTS.md (27 KB) linked — JMeter over test-jmeter/ plus the Kubernetes analytics suite; no overlap found, so this is a linking job
• docs/testing/API_TESTING.md (21 KB) linked — Postman (dotcms-postman/) and Karate (test-karate/); read in full, no overlap with REST_API_PATTERNS.md or INTEGRATION_TESTS.md, so this is a linking job, not M3's
• docs/core/SDK_BREAKING_CHANGE_CATEGORIES.md (13 KB) linked
• The four unowned orphans resolved per M0's ownership decision: docs/cli/CLI_OVERVIEW.md, docs/cli/CLI_BUILD_SYSTEM.md, docs/claude/GPG_COMMIT_SIGNING.md, docs/test-cases/README.md
• docs/integration/API_CONTRACTS.md linked into CLAUDE.md's navigation — already agreed as jointly owned in #37129, where the linking itself was left open as mechanical work
• docs/README.md rewritten as a complete index covering every file under docs/, grouped by area, each entry with a one-line description of when to load it
• Every new link verified to resolve (no typos in relative paths)
• Reachability check automated and committed — a script that walks the link graph from root CLAUDE.md and docs/README.md and fails on any unreachable .md under docs/, so the index can't silently rot again. This is what makes the Rock's third condition checkable instead of a one-time sweep.
• Reachability check green: zero orphans across docs/backend/, docs/testing/, docs/core/ and the rest of docs/

Additional Context

Baseline measured on main at dfbe40f29a: 52 .md files under docs/, 45 reachable, 7 orphaned. Zero orphans remain in docs/backend/ and docs/infrastructure/ — Jose's M2 closed those.

Correction

An earlier revision of this issue asserted that API_TESTING.md overlapped REST_API_PATTERNS.md and INTEGRATION_TESTS.md. That was inferred from the filename, not from reading the file — the same mistake M0's criteria warn against. Both orphans have since been read: API_TESTING.md covers Postman and Karate, PERFORMANCE_TESTS.md covers JMeter, and all three directories they document (dotcms-postman/, test-karate/, test-jmeter/) exist. Neither is a consolidation candidate. Their content still needs the M1 audit against real source, which is tracked there.

Priority

Medium