Use code snippets from managed identity credential samples in identity readme doc and fix minor generation issues

[ahsonkhan]

Fixes #6017

• Updated the snippet generation doc to show a concrete example of the command
• No more extra new lines at eof for generated readmes
• No more extra indentation for code snippets, but we maintain the relative indentation for nesting within the snippet
• Updated the readme to use code snippets from the managed identity credential samples
• Fixed the keyvault secrets sample and updated its readme to use code snippets (looks like the keyvault secrets snippets introduced in Initial cut at snippet generation tool. Also updated KeyVault Secrets readme to use snippets as a working proof of concept. #5213 were overwritten in samples update for nullable #5574)