Don't require pandoc and sphinx during build because a working pandoc is currently only available on Fedora x86.

Refs #6100
Always install the unprocessed markdown documentation.

Refs #6100
Don't distribute COPYING, README,... twice in the icinga2-doc package.

Refs #6100
Merge branch 'feature/fix-rpm-on-arm-6100' into next

Fixes #6100