docs: fix mocking config note and invalid JSON in elasticsearch-logger examples

[AlinsRan]

What this fixes

Two documentation issues found during a docs audit:

1. mocking — inaccurate constraint description

docs/{en,zh}/latest/plugins/mocking.md state that response_example and response_schema "must not be configured together". That is wrong: the schema uses anyOf (at least one required), so configuring both is valid, and the plugin prefers response_example when both are set:

-- apisix/plugins/mocking.lua
anyOf = {
    { required = { "response_example" } },
    { required = { "response_schema" } },
}
...
if conf.response_example then          -- response_example wins when both set
    response_content = conf.response_example
else
    local output = gen_object(conf.response_schema)

Corrected to: at least one must be configured; if both are configured, response_example takes precedence and response_schema is ignored.

2. elasticsearch-logger — invalid JSON in examples

The Admin API examples had a trailing comma after the only key ("index": "gateway",), which makes the JSON invalid on copy-paste. Removed it (en + zh, two places each).

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change (N/A, docs only)
• I have updated the documentation to reflect this change
• I have verified that the changes pass the existing tests