Rename "Unicode Debug" => "Debug", "Unicode Release" => "Release"
since there is no non-Unicode version.
Also, fix the name in other configs for Git, AppVeyor and in the docs.
Fix#10114, close#10278
1. Add Visual Studio 2017 to Pre-requisites section.
2. Add link to community forum with more details about the build process.
3. Restructure Scintilla build instructions. Move common build steps upper.
4. Add more details about the Developer Command Prompt for Visual Studio.
Fix#9087, fix#5566, close#9192
Don Ho