docs(review-rules): document merge_system_rule in the review rules guide

## Description

`merge_system_rule` is an existing boolean field on a `rule.json` entry. When set, the matched user rule is **combined with** the system rule for that file's language instead of **replacing** it:

```json
{
"rules": [
{
"path": "**/*",
"rule": "Security review: flag hardcoded secrets, unvalidated redirects, and missing authz checks.",
"merge_system_rule": true
}
]
}
```

```
$ ocr rules check src/main/java/com/example/UserService.java
Source: Project (.opencodereview/rule.json)
Pattern: **/*
Rule:
────────────────────────────────────────
## System-Specific Rules (Mandatory)

…contents of java.md…

---

## User-Specific Rules (Mandatory)

Security review: flag hardcoded secrets, unvalidated redirects, and missing authz checks.
────────────────────────────────────────
```

The field has been implemented for a while (`ProjectRuleEntry.MergeSystemRule` → `mergeWithSystemRule`, `internal/config/rules/system_rules.go`), but **the review rules guide never mentions it** — not in any of the five locales. The only place it appears in the whole repo is `skills/open-code-review/SKILL.md`, which most users never read.

The cost of this gap is real: #1132 was filed asking for exactly this capability as a new feature, because the docs give every indication that a catch-all user rule unavoidably discards the per-language system rules.

## Scope

- File(s):
- `pages/src/content/docs/en/review-rules.md`
- `pages/src/content/docs/zh/review-rules.md`
- `pages/src/content/docs/ja/review-rules.md`
- `pages/src/content/docs/ru/review-rules.md`
- `pages/src/content/docs/ko/review-rules.md`
- Reference implementation: `internal/config/rules/system_rules.go` — `ProjectRuleEntry` (field declaration) and `mergeWithSystemRule` (merge format and empty-string edge cases)
- Existing wording to borrow from: `skills/open-code-review/SKILL.md`
- Docs-only change; no Go code should need to be touched.

## Acceptance Criteria

- [ ] The "Rule file format (layers 1–3)" section lists `merge_system_rule` alongside `path` and `rule`, stating that it is optional and defaults to `false` (replace).
- [ ] A worked example shows the JSON plus the resulting `ocr rules check` output, so the `## System-Specific Rules (Mandatory)` / `## User-Specific Rules (Mandatory)` structure is visible.
- [ ] The docs state that the system half is resolved **per file**, so one catch-all entry yields `java.md` for `.java`, `python.md` for `.py`, and `default.md` for unrecognized extensions.
- [ ] The docs state that it works in all three user layers: `--rule`, `/.opencodereview/rule.json`, and `~/.opencodereview/rule.json`.
- [ ] The docs state the limitation: it merges the **system** layer only. Multiple *user* entries matching the same file still resolve first-match-wins (see discussion #633), and a matching layer still shadows lower user layers.
- [ ] A recipe is added under "Recipes" for the common case — global security rules on top of the built-in per-language rules — since that is what people search for.
- [ ] All five locales updated with equivalent content.
- [ ] `make test` passes.
- [ ] `make check` passes.

## Context

Discovered while triaging #1132 ("Support adding global review rules"), whose entire request is already satisfied by this flag.

Behaviour verified on `v1.11.0` with `ocr rules check`: `a.java`, `a.py`, `a.go`, and an unrecognized extension each produced a different system half, confirming per-file language resolution rather than a fixed `default.md`; and dropping `merge_system_rule` reduced the output to the user rule alone.

Good first issue: no Go changes required, the behaviour is easy to observe locally with `ocr rules check`, and the surrounding doc already has the right structure — a field list to extend and a "Recipes" section to append to. Note that `pages/src/content/docs/**` is translated prose and is exempt from the English-only scan (see `AGENTS.md`), so the non-English locales should be written in their own language.

Start with pages/src/content/docs/en/review-rules.md, the four locale files, and the wording in skills/open-code-review/SKILL.md; consult internal/config/rules/system_rules.go for merge behavior and edge cases. Update the field list, example, per-file resolution notes, layer limitations, and Recipes section in all locales, then run make test and make check.

Written by the indexing model from the issue text.