Add Customer Stories

[t4niwa]

Description

This PR adds a new "Customer Stories" section to the Athenz website, featuring LY Corporation's implementation of Athenz in their Flava IAM platform.

• https://tech-verse.lycorp.co.jp/2025/ja/session/1040/

[ctyano]

@t4niwa I wondered that the introduction to “Flava IAM: Bridging Security and Usability with Athenz” appeared a bit abrupt.
The sentence starting with “In this session~” seems to have been copied directly from Tech-Verse 2025 Session: Flava IAM: Bridging Security and Usability with Athenz, however, within the context of the Athenz open-source website, I feel there should be a brief introduction explaining what this is.
I believe a step-by-step explanation is necessary: first, briefly introduce the company LY Corporation is using Athenz; second, explain that LY Corporation has built a Private Cloud called Flava(maybe including why we did so); and third, present an example of how Athenz is utilized within that Flava.