Add subfolder creation instructions to allas-ui.md

[DeRuina]

Added instructions for creating subfolders inside a bucket.

Allas-UI docs:

• https://csc-guide-preview.2.rahtiapp.fi/origin/DeRuina-patch-1/data/Allas/using_allas/allas-ui/#creating-subfolders-inside-a-bucket

What's New entry:

• https://csc-guide-preview.2.rahtiapp.fi/origin/DeRuina-patch-1/support/wn/cloud-new/#allas-web-ui-improvements-14112025

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• I have followed the instructions in the Contributing and Styleguide documents.
• My pull request passes the automated tests. If not, visit pull requests at Travis CI and have a look at the job log.
• I have included a link to the appropriate preview page (select your branch from the list).

[jemaltahir]

LGTM