i18n(ja) JA Translations for newly added files in July/August 2025

[J1-takai]

Description

i18n(ja): Add Japanese texts for the following two new files:

• project-structure.mdx in Quick Start section (Aug 1, 2025)
• "App Icons": icons.mdx in Develop section (Jul 21, 2025)

@vasfvitor - Will you ask someone to check the following:
In Quick Start Menu, the newly added "Project Structure" file seems to be a continuation from the "Create a Project" file but these two files have the almost same ##Next Steps section; In the Create a Project file, there are file links, the first one of which are the link to the newly added "Project Structure" file. The other 4 links are already copied in the "Project Structure" file. So I think those 4 links are duplicated and redundant in the "Create a Project" file?

Thank you always for your support on my PRs.

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[vasfvitor]

the links are duplicated but I don' think it is an issue

[vasfvitor]

btw, you got it working on first try, no fixing needed :)

[J1-takai]

@vasfvitor thank you for your quick approval. 👍

• the links: yes, they will cause no issues. It's just "cosmetic" matters of documentation.
• no fixing: I am glad to know this PR did not cause you an extra trouble. I do not know what brought such a windfall though.

When you find some spare time, please also help my old PRs #3336, #3379, #3399, and #3445. Once merged, we can catch up the Chinese Documentation team!