Expose more information when `DocService`'s debug menu is unavailable
Depending on the available transport types, sometimes DocService's debug menu might be unavailable.
Currently, we simply hide the Debug menu without any explanation.
For instance, the debugTransport variable depends on MimeType
https://github.com/line/armeria/blob/8ac54ee4cfa067fc96bf7a35d3f3896d1b345ff6/docs-client/src/containers/MethodPage/index.tsx#L194-L194
https://github.com/line/armeria/blob/8ac54ee4cfa067fc96bf7a35d3f3896d1b345ff6/docs-client/src/containers/MethodPage/index.tsx#L150
If the appropriate MimeType isn't available, the Debug menu isn't visible
https://github.com/line/armeria/blob/8ac54ee4cfa067fc96bf7a35d3f3896d1b345ff6/docs-client/src/lib/transports/index.ts#L31
It might be worth adding to the UI why the Debug menu is unavailable. (what the current MimeType is, and the supported MimeTypes)
ref: https://line-armeria.slack.com/archives/C1NGPBUH2/p1684394002523209
We could also give a link to the documentation on how to configure GrpcService to make debug forms work.