docs(15.9): document installing plugins from the command line

[marevol]

Describes codelibs/fess#3432, which is not merged yet. The <name>:<version>
form does not exist in a published build until it is.

Problem

admin/plugin-guide.rst only covers the admin UI, so nothing in the docs says
how to install a plugin at a chosen version. Without one, bin/fess-setup asks
the repository for the newest version built for this Fess, so a build step such
as a Dockerfile installs something different on each run.

Change

A new section, Installing from the Command Line, in the 15.9 tree of all
seven languages (ja / en / de / es / fr / ko / zh-cn). It covers:

• bin/fess-setup install plugin <name> and what happens without a version
• pinning with <name>:<version>
• that plugins are released separately, so each one needs its own version
  rather than a single shared value
• --version as the default for the names that carry none
• that the previously installed version is deleted after the new one is
  installed, and that a version that does not exist exits 1, so a Dockerfile
  fails rather than carrying on with the plugin missing

The section sits at the same level as the existing management section; the
admin UI text is unchanged.

Verification

Each file parsed with docutils (doctitle_xform=False, the |Fess|
substitution supplied as the build's rst_prolog does) and compared against
the same file on master:

	sections	system messages	new messages	raw markup left in text
ja	1 → 1	4 → 4	none	none
en	1 → 1	0 → 0	none	none
de	1 → 1	0 → 0	none	none
es	1 → 1	1 → 1	none	none
fr	1 → 1	0 → 0	none	none
ko	1 → 1	2 → 2	none	none
zh-cn	1 → 1	2 → 2	none	none

The remaining messages are the too-short heading underlines already in these
files and are left alone. The new headings' underlines are generated from the
display width, so CJK titles are not short. The resulting tree puts the new
section as a sibling of the management section in every language.