[CADL API view] Improve CADL instructions for upload

[catalinaperalta]

While working with a service partner recently it became clear that the instructions for uploading an APIView for CADL should be improved.

Suggested instructions:

1. Install @azure-tools/cadl-apiview
2. Compile and emit the apiview file: cadl compile .\main.cadl --emit "@azure-tools/cadl-apiview"
3. Upload the JSON file that is emitted.

[catalinaperalta]

cc @maririos @lmazuel @praveenkuttappan

[catalinaperalta]

Another improvement could be to make it more obvious that a CADL or JSON file is accepted. It would be good if the whole screen didn't go to an error when the JSON API view token file is uploaded or the space is left empty. A more descriptive error could help in understanding what the problem is.

Same thing happened when adding a revision, it was not immediately obvious that the dropdown should be used to be able to upload the CADL token file as a JSON. When the dropdown is set to Cadl there is no button to upload a file since it is expecting a URL and it is not intuitive to change the dropdown option from Cadl since the API view is for a Cadl spec.

In this case we decided to simply not choose a language to get the instructions or to upload since that seemed to be the problem. Initially we were unable to add a revision as well.

[catalinaperalta]

Instructions as to what URL to use in the API view instructions section would also be appreciated. If the CADL is in PR, should it be a link to the main.cadl file in that PR?