Update occurrences of data frame transform terminology

[lcawl]

The terminology for "data frame transforms" is changing in 7.x and the documentation must be updated accordingly. It is acceptable for these terminology changes to be back-ported all the way to 7.2 documentation.

• Update the shared attributes in https://github.com/elastic/docs/blob/master/shared/attributes.asciidoc: Changes data frame transforms to transforms in the attributes docs#1139
• Update the occurrences (that don't use shared attributes) in the stack-docs repo: [DOCS] Updates data frame transform terminology #517
• Update the occurrences (that don't use shared attributes) in the elasticsearch repo: [DOCS] Updates dataframe transform terminology elasticsearch#46642
• Update the occurrences (that don't use shared attributes) in the kibana repo: [DOCS] Updates data frame transform terminology kibana#45967
• Update anchors in file URLs: [DOCS] Update data frame transform URLs elasticsearch#46940
• Update links to new URLs: [DOCS] Fixes link to transform APIs #525, Update transform API URLs kibana#46260, Add redirects for transform APIs docs#1203