Redirect `diffblue.github.io` to `diffblue.github.io/cbmc`

[NlightNFotis]

In #7152, there was a suggestion by Peter to redirect URL requests from https://diffblue.github.io to https://diffblue.github.io/cbmc, the root directory for CBMC documentation.

@tautschnig did some research on that, and according to the resource he cited there (https://inbo.github.io), we need to make a new repository, diffblue.github.io, and include some html code that redirects to that specific endpoint.

Making a note of this here, so that it doesn't get forgotten.