docs(install): correct the 15.8 upgrade guide against the implementation

[marevol]

Summary

Reviewed 15.8/install/upgrade.rst against the actual implementation (fess, fess-parent, docker-fess, corelib) and fixed what it had carried over unchanged from 15.7. Applied to all seven languages.

Most of these are copy-paste survivors: create_version.sh rewrites the Fess version token but not embedded dependency versions or procedures, so the page had drifted from the code.

Corrections

Issue	Was	Is
Release archive	fess-15.8.0.tar.gz + tar -xzf	fess-15.8.0.zip + unzip
OpenSearch	3.7.0 (4 plugin installs + 2 prose spots)	3.8.0
Docker volume backup	-v search01_data:/data	-v ${PROJECT}_search01_data:/data
/etc/fess/* on upgrade	"automatically preserved"	RPM only; DEB overwrites it
fess_basic_config.bulk	"basic settings"	19 config indices
fess_config.bulk	"crawl settings, scheduler, labels…"	superset of 25 indices
fess.json / doc.json	one item, "index settings (mapping)"	settings vs. document mapping

Evidence:

• tar.gz does not exist. targz-bin.xml has been commented out in fess/pom.xml since 2015; only zip-bin.xml is active. The fess-15.7.0 release carries exactly .deb, .rpm, .zip. The documented download was a 404.
• OpenSearch 3.8.0. fess-parent/pom.xml declares <opensearch.version>3.8.0</opensearch.version>, and fess/plugin.xml pins analysis-fess, analysis-extension, minhash and configsync at 3.8.0 (plus knn at 3.8.0.0). Installing 3.7.0 plugins would fail the version check the page itself warns about.
• The volume backup silently produced nothing. Compose prefixes volume names with the project name, which the page states two lines earlier. docker run -v search01_data:/data therefore creates a new empty volume, exits 0, and writes an empty tarball that looks like a successful backup.
• DEB does not preserve /etc/fess. src/packaging/deb/scripts/conffiles lists only /etc/default/fess, /etc/init.d/fess and the systemd unit, so dpkg -i overwrites fess_config.properties. RPM does preserve it via <configuration>noreplace</configuration>.
• The two bulk descriptions were swapped. fess_basic_config is an alias over 19 config indices; fess_config is a superset of 25 that adds coordinator, crawling_info, crawling_info_param, failure_url, job_log and thumbnail_queue.
• Step 3 never restored fess_config.properties or log4j2.xml even though step 1 told you to back them up.

Additions

• k-NN is mandatory in 15.8. fess_indices/fess.json always sends "knn": true and knn.derived_source.enabled, and doc.json always declares content_chunk_vector, regardless of whether chunk vector search is enabled. Index creation fails without the plugin. Step 4 is no longer labelled "if necessary".
• Upgrading from 14.x crosses an OpenSearch major version (14.19 shipped 2.19.0).
• Restore accepts only *.bulk, system*.properties, gsa*.xml, fess*.json and doc*.json; the four *.ndjson log files are rejected. Uploading an old fess.json/doc.json overwrites the shipped index definitions, and an uploaded system.properties is loaded into memory only and lost on restart.
• A Docker rollback procedure, which was missing entirely.
• Per-install-method log paths (logs/fess.log, /var/log/fess/fess.log, docker compose logs), plus fess-crawler.log / audit.log / searchlog.log.
• 15.8 migration work: fess-webapp-semantic-search removal and the fess-llm-* split.
• bin/fess -p writes a pid file; no stop script ships with the archive.
• Replaced the Upgrade Guide -> https://fess.codelibs.org/ja/ link, which was circular (this page is the upgrade guide) and pointed at the Japanese site from the English, Spanish and French pages, with :doc:prerequisites``.

Verification

• All 7 files parse with zero docutils warnings. This required padding the CJK heading underlines in ja and zh-cn, which were emitting real Title underline too short warnings (docutils counts CJK as two columns); install-docker.rst and install-linux.rst were already normalized.
• Structural parity across all 7: 35 headings in the same order and level, 63 literal blocks, 18 notes, 8 warnings, 2 important.
• All technical literals (versions, paths, flags, property names, :doc:/:ref: targets) are byte-identical across languages.
• No 3.7.0 and no fess.codelibs.org/ja/ remain in any language.

Follow-up (not in this PR)

3.7.0 is still stale in six sibling pages — prerequisites, install, install-linux, install-windows, run, uninstall — across all 7 languages. Worth a mechanical sweep so the install set stays self-consistent with this page. Two other pre-existing errors surfaced during review: install-linux.rst misquotes FESS_DICTIONARY_PATH as /var/lib/opensearch/data/config/ (the shipped value is /var/lib/opensearch/config/), and config/crawler-advanced.rst writes the crawler log as fess_crawler.log (the real name is fess-crawler.log).