[docs] Update installation docs to include Fedora

- All the instructions mentioned for CentOS/RHEL are also valid for Fedora Linux so add it.
- The package name for atlas is atlas-devel not libatlas-devel for CentOS/RHEL/Fedora so fix it.
- Add 'sudo' prefix to missing places to be consistent with the rest of the document.
- Add instructions for installing python headers in Fedora into the python support section.
- Update Linux installation section to reflect Fedora, refactor a bit.