docs: Add Japanese translation to docs

[tobybellwood]

This PR adds the first translation into Japanese for the documentation.

This translation is correct as at the Lagoon v2.19.0 release

It's currently missing:

• Full support for white-label docs variables (docs: Allow white-label and internationalise Lagoon docs #3703)
• Any docs changes since 2.19.0
• The screenshot images are still of English language apps
• Some formatting may be slightly strange
• Release notes in the docs aren't translated into Japanese
• Documentation on how to provide a translation (coming soon)

The initial documentation conversion was done by @stqp - thanks so much!!!

[AlannaBurke]

Tried to click through every page, found a few broken bits:

• Should any of the code snippets be translated? I noticed some of .lagoon.yml has translated snippets. (http://127.0.0.1:8000/ja/concepts-basics/lagoon-yml/#example-lagoonyml)
• also the pygmy install command: http://127.0.0.1:8000/ja/using-lagoon-the-basics/local-development-environments/
• bottom of this page got weird, should be two different snippets: http://127.0.0.1:8000/ja/using-lagoon-advanced/simplesaml/
• snippet got broken here: http://127.0.0.1:8000/ja/installing-lagoon/lagoon-logging/
• There's a broken callout note on this page: http://127.0.0.1:8000/ja/installing-lagoon/opendistro/
• broken button: http://127.0.0.1:8000/ja/installing-lagoon/add-project/
• first bit of code here should be a block: http://127.0.0.1:8000/ja/installing-lagoon/lagoon-backups/

[AlannaBurke]

lgtm!