Ckeditor5: Improve MathType's and Spell checker's guide URL

[Reinmar]

They'll go like this now:

• Math equations and chemical formulas – https://ckeditor.com/ckeditor5/build/docs/ckeditor5/latest/features/mathtype.html
• Spelling and grammar checking – https://ckeditor.com/ckeditor5/build/docs/ckeditor5/latest/features/spell-checker.html

In both cases the guide's title is much better than the URL. This is especially bad in the MathType's case.

[Reinmar]

I'd go with:

• math-equations.html
• spelling-and-grammar-checking.html

WDYT?