docs: deprecate `@angular-devkit/build-optimizer` (#43915)
docs: deprecate `@angular-devkit/build-optimizer` (#43915)

The entire package is deprecated in v13. Deprecation message is mostly copied from the package's [README](https://github.com/angular/angular-cli/blob/13.0.0-next.9/packages/angular_devkit/build_optimizer/README.md), with just some minor edits.

(cherry picked from commit d81744e60018281e69f98c051e139f9336127b8e)

PR Close #43915

angular/angular,gnomeontherun/angular,angular-indonesia/angular,gnomeontherun/angular,shairez/angular,angular-indonesia/angular,brandonroberts/angular,angular-indonesia/angular,brandonroberts/angular,angular/angular,gkalpak/angular,brandonroberts/angular,manekinekko/angular,brandonroberts/angular,gnomeontherun/angular,ocombe/angular,JiaLiPassion/angular,ocombe/angular,angular/angular,manekinekko/angular,JiaLiPassion/angular,shairez/angular,shairez/angular,shairez/angular,brandonroberts/angular,JiaLiPassion/angular,manekinekko/angular,JiaLiPassion/angular,ocombe/angular,gnomeontherun/angular,manekinekko/angular,ocombe/angular,angular-indonesia/angular,ocombe/angular,gkalpak/angular,gkalpak/angular,brandonroberts/angular,angular/angular,manekinekko/angular,gkalpak/angular,shairez/angular,ocombe/angular,manekinekko/angular