Add a new japanese conference video (#4000)
Add a new japanese conference video (#4000)

Motivation:

Add a new japanese conference video. 
I made a presentation about Armeria integration of LINE STORE at the Japanese conference, [Spring Fest 2021](https://springfest2021.springframework.jp/) this month, and then the youtube video is posted. 
I would like to share it as the community slide and video.

Modifications:

- Add a new `Article` tag for it.

Result:

- The `Article` is shown in the following image.
![スクリーンショット 2021-12-24 15 41 21](https://user-images.githubusercontent.com/20625903/147325765-c5042a16-a3a8-4ab6-a0bc-a52af4801895.png)
trustin/armeria,minwoox/armeria,trustin/armeria,kojilin/armeria,kojilin/armeria,line/armeria,line/armeria,minwoox/armeria,kojilin/armeria,line/armeria,trustin/armeria,kojilin/armeria,minwoox/armeria,minwoox/armeria,trustin/armeria,trustin/armeria,line/armeria,minwoox/armeria,minwoox/armeria,line/armeria,trustin/armeria,kojilin/armeria,kojilin/armeria,line/armeria