Migrate to new repository

[awvwgk]

We decided that we will create a separate repository for the new sphinx based webpage at https://github.com/fortran-lang/webpage. This allows to keep this repository and maybe also the current page at https://old.fortran-lang.org or similar.

We expect to make the new page the default latest by end of August 2022.

[awvwgk]

We plan to finish the migration by end of next week. Next step is to add a banner to the current page to make visitors aware of the upcoming migration.

Once we migrated to the sphinx based webpage this repository will be available at the default GH pages URL https://fortran-lang.github.io/fortran-lang.org and we will create a banner on the new webpage referencing the old repository in case somebody finds something missing.