docs: add changelog for 4.2.1

docs: add changelog for 4.2.0-rc.2

docs: add changelog for 4.2.0-rc.1

docs: add changelog for 4.2.0-rc.0

docs: add changelog for 4.1.3

docs: add changelog for 4.1.2

docs: add changelog for 4.2.0-beta.0

docs: add changelog for 4.1.1

build(aio): convert triple backtick blocks to `code-example` tags

Closes angular#16246

docs: add changelog for 4.1.0