Fix documentation to not use multiple types

[s1monw]

There are still some places in the documentation that should be fixed to either use the new parent join facilities or should be fixed in general to not use multiple types:

• reference/aggregations/bucket/children-aggregation.asciidoc
• reference/docs/index_.asciidoc
• reference/indices/put-mapping.asciidoc
• reference/mapping/dynamic/default-mapping.asciidoc
• reference/mapping/fields/parent-field.asciidoc
• reference/mapping/fields/type-field.asciidoc
• reference/mapping.asciidoc
• reference/query-dsl/parent-id-query.asciidoc

some of them might even be removed. Relates to #24961