[doc] client : ilogtail loki integration (#8970)

**What this PR does / why we need it**:
https://github.com/alibaba/ilogtail/pull/685

ilogtail is a very popular log agent in China, with tens of millions of
installations. The open source community of ilogtail has completed the
integration of loki. This PR is completed as a supplement to loki doc

demo config，Type: flusher_loki
```yaml
enable: true
inputs:
  - Type: file_log
    LogPath: .
    FilePattern: simple.log
flushers:
  - Type: flusher_loki
    URL: http://localhost:3100/loki/api/v1/push
    ExternalLabels:
      source: ilogtail
```

**Which issue(s) this PR fixes**:
Fixes #<issue number>

**Special notes for your reviewer**:
flusher_loki PR snapshot

![image](https://user-images.githubusercontent.com/9583245/229053024-66fcd286-a2f8-4a32-9ec5-758b63554986.png)

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [x] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`

docs/sources/clients/_index.md

@@ -77,3 +77,4 @@ when using or writing a third-party client.
 - [mjaron-tinyloki-java](https://github.com/mjfryc/mjaron-tinyloki-java) (Java)
 - [LokiLogger.jl](https://github.com/JuliaLogging/LokiLogger.jl) (Julia)
 - [winston-loki](https://github.com/JaniAnttonen/winston-loki) (JS)
+- [ilogtail](https://github.com/alibaba/ilogtail) (Go)