docs: Installation Instruction Title & Description Fixes
Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)

1. All titles are listed by simple platform names apart from Windows' "Installing Docker On Windows". Changed this to "Windows" to match the rest.

2. Some articles' description (and title) information does not match the majority. Modified them to match the rest (i.e. Please note this project is currently under heavy development. It should not be used in production.)

3. Removed "Linux" from Gentoo & Ubuntu descriptions.
Merge pull request #4019 from ostezer/fix-installation-windows-doc-title

docs: Installation Instruction Title & Description Fixes