Document unicode normalization

[AlexDaniel]

It is already mentioned here:

• https://docs.perl6.org/language/unicode#File_Handles_and_I/O
• https://docs.perl6.org/language/unicode#UTF8-C8
• https://docs.perl6.org/language/traps#All_Text_is_Normalized_By_Default

But it feels like something is lacking. What purpose does it serve? Why do we do it? Maybe some links to unicode.org? Maybe some examples?

[samcv]

What section should this be added to, or do we need a new section?

[AlexDaniel]

I think right at the top would be nice. Normalization sounds ok as a heading. Maybe needs a word or two about graphemes also, possibly just copypasted from c0159cac65#diff-5ea33ed4c36cc3533068127d13fa490fR714.

FWIW there's also https://docs.perl6.org/language/traps#All_Text_is_Normalized_By_Default (ah, this was already mentioned)