Clean up azure-identity README (#48685)

* Clean up azure-identity README

- Fix typo: 'used by to' -> 'used to'

- Remove redundant DefaultAzureCredential and ManagedIdentityCredential code examples

- Update credential chain and managed identity reference links

- Improve Markdown table formatting

- Fix indentation and whitespace issues

* Update sdk/identity/azure-identity/README.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: scottaddie