docs(15.9): follow the removal of the JSP search UI and the JAR theme type

[marevol]

Summary

Bring the 15.9 documentation in line with codelibs/fess#3497, which removed the JSP search UI and the fess-theme (JAR theme) plugin type.

In 15.9, saving the General settings no longer copies JSP files into WEB-INF/view/<virtual host>/, the login page (/login/) is served from WEB-INF/view/admin/login/ for every virtual host, and a fess-theme-*.jar is kept only as a plain jar that changes nothing. Several pages still described the old behavior.

Changes

All seven languages, 15.9 tree only (10 files per language):

• config/security-virtual-host.rst: no JSP files are generated; the login page is shared by all virtual hosts; a virtual host's look comes from the static theme named after it.
• dev/theme-development.rst: the JAR theme plugin section is replaced by a short "removed in 15.9" section. It says what happens to such a jar (installed as a plain jar, removable from the plugin page or with fess-setup remove plugin), that fess-setup no longer lists or installs them, and how to move to a static theme. The JAR theme examples are dropped from the list of existing themes.
• dev/plugin-architecture.rst: fess-theme-* is removed from the plugin type table and the published plugin list, with a note pointing to static themes.
• api/admin/api-admin-plugin.rst: the type values no longer include fess-theme, and now list fess-storage and fess-sso (the ids in PluginHelper.ArtifactType).
• admin/theme-guide.rst: the JAR theme note says the type was removed.
• dev/webapp-plugin.rst: the replacement warning no longer tells readers to copy addDesignJspFileName calls (the method is gone, and a replacement that still calls it fails with BeanMethodNotFoundException); the search-screen section points only to static themes.
• config/load-control.rst: a web request over the limit gets the theme's error page, not busy.jsp.
• dev/overview.rst: WEB-INF/view/ now holds the admin and login JSPs only.
• user/role-search.rst: after a password change the user is logged out and logs in again; the page is the theme's /profile, which calls POST /api/v2/auth/password.
• install/upgrade.rst (15.8 to 15.9, static theme section): per-virtual-host login pages and JAR theme plugins are gone. Custom fields shown in the JSP results must also be added to query.additional.api.response.fields, since the static theme reads results from /api/v2/search and that API returns only the fields on its allow-list.

Checks

• python3 tools/check_headings.py on every changed file: no mismatches
• Sphinx HTML build of the 15.9 tree for each of the seven languages, before and after: no new warnings or errors