docs: organize Japanese translations under docs/ja/

[atani]

Summary

• Move Japanese translations (docs/*.ja.md) into a dedicated docs/ja/ folder and drop the .ja.md suffix
• Fill in the missing Japanese translations: redash-guide and sharing-connections
• Add a pointer to docs/ja/ from the README Documentation section

Background

The docs/ folder had a mix of English originals and .ja.md translations, and only two out of four guides had a Japanese version. Separating English and Japanese into sibling folders makes the structure self-evident and makes it obvious when a translation is missing (the file simply does not exist on the Japanese side).

Changes

docs/getting-started.ja.md   →  docs/ja/getting-started.md
docs/import-guide.ja.md      →  docs/ja/import-guide.md
docs/ja/redash-guide.md      (new)
docs/ja/sharing-connections.md (new)

The English files under docs/ are unchanged.

Test plan

• docs/ja/ contains 4 Japanese files at parity with the English originals under docs/
• README link to docs/ja/ renders correctly on GitHub
• Existing README links to English docs still resolve (unchanged)