[Docs] Docs improvements

- We'll discuss with docs team to update https://docs.microsoft.com/en-us/cli/azure/azure-cli-extensions-list more timely.
- Improve development documentation in https://github.com/Azure/azure-cli/tree/dev/doc/extensions, for example, guidance for multiple api version support, `experimental` flag support levels (extension, command group, command and argument).

GA, Preview and Experimental: https://docs.microsoft.com/en-us/cli/azure/reference-types-and-status#what-is-reference-status