Cadl doc preview support

[ruowan]

1. Support doc preview for Cadl PR in spec repo
2. Doc auto refresh v2 support cadl

[live1206]

1 is supported already since swagger files are still kept in Cadl PR

[live1206]

For 2, learned from SDK generation that, the entry Cadl file should import/reference to all needed Cadl files, so that the related services will be generated as well during compilation.
Need to test if Cadl compilation generates multiple swagger files from entry Cadl or just 1.
And also, there is no multiple versioning support for Cadl + Dpg, there is just 1 version including all API versions right now.

So, there is no way to rely on SDK version/tags like before with readme files, we can only rely on API versions with Cadl files.

[live1206]

In conclusion, for Cadl + Dpg, we can only leverage API version since there is no SDK version anymore.
We will get latest/latest stable version from API version for Docs publish.
cc @josefree