Arabic: Fix, improve, and update the translation (#899)

* Fixes and improvements on the Arabic translation

Translation corrections; removing editing residual (repetitions and misplaced words); more fluent wording and phrasing; fixing typos; more consistency; inserting English names of institutions for reference; and other minor fixes, including avoiding Latin words qualified by the Arabic article.

* Update CONTRIBUTING.ar.md

Fixing several technical mistranslations; updating the text: Arabic seems to refer to an earlier versoin of the English; some syntactical fixes