Remove duplicated documentation from this repo

[awvwgk]

Now that we have a dedicated repository for the fpm documentation we should try to minimize the duplication and overlap of different information sources.

• move everything from the ford pages section: https://fortran-lang.github.io/fpm/page/index.html
• move markdown documents:
  • https://github.com/fortran-lang/fpm/blob/main/PACKAGING.md
  • https://github.com/fortran-lang/fpm/blob/main/manifest-reference.md
    available at: https://fpm.fortran-lang.org/en/spec/manifest.html
• check wiki for salvageable documentation: https://github.com/fortran-lang/fpm/wiki

[Divyansh200102]

@awvwgk is this issue still open and can you please tell how can I resolve this issue?