docs(locale): add Japanese catalogs for 25 chapters and index

[sanak]

Overview

Groundwork for bringing the Japanese translation to 100%. This PR places the .po
catalogs for all 26 message catalogs under locale/ja/LC_MESSAGES/ and applies
mechanical corrections only. It contains no translation work — that comes in PR #2.

This branch is kept as the upstream-sync branch. When upstream's .pot files are
updated, this branch is rebased, msgmerge is run here, and the result is propagated
to update-locale-ja.

Changes

• Place .po catalogs for all 26 catalogs, i.e. 25 chapters plus index
  (10 existing catalogs updated, 16 newly added)
  • The source pot files were verified byte-identical to locale/pot, so there is
    no drift from the English source
  • This puts the 163 already-translated entries of basic/graphs under version control
• Fix terminology and markup violations (check_terms.py exits 0 for all 26 catalogs)
• Resolve 14 fuzzy entries

Result of the rescue pass

Checked whether placing these catalogs dropped any previously translated strings.
Five messages exist in the Git version but not in the new one, and all five are
msgid-gone — the English source string itself was removed or changed. None of
those msgids appear anywhere in the current locale/pot. Zero translations need
to be restored.

Current statistics

290 translated / 0 fuzzy / 1,171 untranslated / 1,461 total (summed per file)

Verification

• msgfmt -c passes for all 26 catalogs
• check_terms.py exits 0 for all 26 catalogs
• 0 fuzzy entries
• No msgid was modified (verified with git diff)
• No .mo files are committed
• make html-ja completes and generates 25 HTML pages (same count and structure as en)
• basic/graphs.html renders its heading in Japanese
• No raw substitutions such as |place_N| are left in the generated HTML

Known issues (deliberately not addressed in this PR)

• The Project-Id-Version header of the 16 newly added catalogs mixes workshop
  generations (Belém 3.0 / Auckland 3.1 / Hiroshima 3.2). This is inherited from the
  files they were copied from.
• The same 16 catalogs still carry sphinx-intl's default PO-Revision-Date and
  Last-Translator values. That is normal for untranslated catalogs; whether to
  update them will be decided when the translation is complete (PR docs(locale): translate all remaining chapters into Japanese #2).

[sanak]

Close this PR, because upstream merged the following PR.
pgRouting#284