Migrate technical articles in fluentd-website

[fujimotos]

There are still a number of technical articles left in the main website.

https://www.fluentd.org/guides

First, we need to maintain these articles up-to-date. Most of code examples in the article still rely on v0.12 series and need migrating to Fluentd v1.

In the process, we will migrate these articles to docs.fluentd.org. The new home will be under the "Use Cases" category:

https://docs.fluentd.org/v1.0/articles/free-alternative-to-splunk-by-fluentd