chore(macros/LearnSidebar): add Japanese translation MathML section

[gurezo]

Summary

@hmatrjp @potappo , mdn-yari-ja

please review for this pull request

• issue: サイドメニュー の翻訳(MathML) #785

Problem

• There is no Japanese translation yet

English pages

• cf. https://developer.mozilla.org/en-US/docs/Learn/MathML
  • MathML — Writing mathematics with MathML
    • MathML_first_steps': 'MathML first steps'

Japanese pages

• cf. https://developer.mozilla.org/ja/docs/Learn/MathML
  • MathML — MathML を使用して数学を記述する
  • MathML の最初のステップ

before	after

Solution

translate japanese for LearnSidebar.ejs

NOTICE

@mdn/core-dev

Please merge once mdn-yari-ja is approved.

Before

• There is no Japanese translation yet

After

• tranlate japanese for LearnSidebar.ejs

[hmatrjp]

Looks good.