docs: add the 15.9 documentation set for all seven languages

[marevol]

Fess master is now 15.9.0-SNAPSHOT, so the documentation needs a 15.9 tree to receive the changes made during that development cycle.

What this does

Created with ./create_version.sh 15.9, which copies the 15.8 tree for ja, en, de, fr, es, zh-cn and ko along with the ja and en image directories, and rewrites the documentation version token in the copied RST files. 1315 RST files and 192 images.

versions.json moves development to 15.9. release stays at 15.8, and neither supported nor schedule changes: 15.9 is not released, so it has no support window and no end-of-life date yet. This is the same shape versions.json had while 15.7 was the release and 15.8 was in development. The generated eol.rst tables are unaffected, since they are built from supported and schedule.

Verification

• Each 15.9 tree has exactly the same files as its 15.8 counterpart — no file added, dropped or renamed.
• Every one of the 2127 changed lines differs from its 15.8 original only by the version token, so none of the number corruption fixed in docs: add 15.8 and fix version-bump number corruption (15.1-15.8) #428 was reintroduced.
• No stray 15.8 reference remains inside any 15.9 tree.
• The OpenSearch version named throughout install/ is 3.8.0, which still matches <opensearch.version> in fess-parent on master, so no dependency version is carried over stale.
• The lint workflow passes locally: python -m unittest discover -s tools, tools/update_eol.py --check, and tools/check_headings.py over the 15.8 and 15.9 directories.

Known, and deliberately left alone

The mechanical rewrite also renamed statements that record what changed in 15.8 — the 15.8 固有の移行作業 / Changes in 15.8 sections and roughly 250 similar lines across 14 files in the seven languages, such as "folded into the core in 15.8" and "the LLM features were split out in 15.8". Those now say 15.9 and are wrong.

Correcting them is a content change that would be invisible inside a copy this large, so it is left to focused follow-up pull requests, the same way 15.8's content was corrected after #428.