Add link to unicode collation docs

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/14/collation.html
Description:

This page should include a link to the Collation docs for the Unicode Locale
Data Markup Language (LDML). 
http://www.unicode.org/reports/tr35/tr35-collation.html#Collation_Settings
If one wants to create a custom collation e.g. de-DE-u-kn-true, only there
are all possible value for creating customized collations.

Kind regards
Dominik