Add troubleshooting documentation for "Permission denied" errors

If you deploy to Linux w/o read permissions to certain platform files like host.json/function.json, you can get an error like this:

`Access to path '/home/site/wwwroot/host.json' is deined. Permission denied.`

Most of our tooling should prevent this, but it is still possible to manually zip files with incorrect permissions.

We need to:
1. Add a note to our troubleshooting docs explaining what this error can mean and which files need to have read or execute permissions.
2. Add a DiagnosticEvent when we detect a permission error on any of the well-known files that we need to read (host.json, function.json). It may be tough to catch every situation, but getting a few in here on the well-known files would help customers self-mitigate.

Locate the troubleshooting documentation and the permission checks for the well-known files host.json and function.json. First review how permission errors are detected and how DiagnosticEvent is emitted; done means the docs explain required read or execute permissions and relevant permission failures produce a DiagnosticEvent.

Written by the indexing model from the issue text.