[translation] improve docs and samples for glossary and custom model scenarios #17918
Description
service instructions for creating a custom model: https://docs.microsoft.com/en-us/azure/cognitive-services/translator/custom-translator/quickstart-build-deploy-custom-model
examples of glossaries: https://machinetranslation.visualstudio.com/MachineTranslation/_git/mt-batch?path=%2Ftest%2FTestFiles%2FTSV%2Fin&version=GBmaster