Add ja translation for double faults

[garasubo]

Resolve #915

Japanese translation for "Double Faults" section

This is my first time PR, so feel free to point out if I'm in the wrong way.

[phil-opp]

Thanks a lot for creating the translation! I'm not speaking Japanese myself, but maybe @JohnTitor or @woodyzootopia could take a look?

[swnakamura]

Thank you very much! I think it's really good. There are only a few advice:

• JohnTitor is added as co-translator since he always corrects my translation. You need not add him as for now. (I think he'll help you as well though!)
• I think it's not usual to use italic characters （イタリック体） to emphasize in Japanese. I'd prefer converting it bold（太字）.
• It would be more kind to change URLs to the websites in English to the ones in Japanese when translation is available (for example, Wikipedia 日本語版 or Rust by Example 日本語版).

There are other minor mistakes, but they can be corrected after all translation is done. Keep going!

[garasubo]

All sections are translated.

TODO: Replace URLs for English website with those for Japanese websites if exist

[garasubo]

Replaced wikipedia links and Rust official document links to those for Japanese. Confirmed that all CI checks passed.
@JohnTitor @woodyzootopia Could you review this PR?

[swnakamura]

Sorry for the delay in response. I've been really busy with my jobs until next week or so, so it's difficult to review your translation for now. Very sorry.

[JohnTitor]

Thanks for translating and sorry for the delay! Left some reviews roughly but almost looks great to me :)

[JohnTitor]

Btw, IIRC "Solve #XXX" won't close the issue, you should use "resolve", "close", or "fix" instead.

[JohnTitor]

Thanks @garasubo!

@woodyzootopia I think it's good to go ahead as-is and follow-up stuff when you get some time to review, does it work for you?

[swnakamura]

Yes, thank you! I'm still not able to check the translation, so please proceed! I will create another PR/Issue if I were to find any problem.

[phil-opp]

Ok great, thanks a lot!