Doc improvements for language tags and custom ICU collations.

Separate the documentation for language tags themselves from the
available collation settings which can be included in a language tag.

Include tables of the available options, more details about the
effects of each option, and additional examples.

Also include an explanation of the "levels" of textual features and
how they relate to collation.

Discussion: https://postgr.es/m/25787ec7-4c04-9a8a-d241-4dc9be0b1ba3@postgresql.org
Reviewed-by: Jonathan S. Katz

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