A user building the C driver from source should not need to look at the instructions for other platforms. I'm thinking of the following layout:

• Installing
  • Supported Platforms
  • Install libmongoc with a Package Manager
  • Install libbson with a Package Manager

And have that top-level installation page link to the platform specific pages:

• Building on Unix
• Building on macOS
• Building on Windows with Visual Studio
• Building on Windows with MinGW
• Advanced Installation Options
  • Additional Options for Integrators

And split the libbson-only instructions into libbson pages. We can reduce duplication with includes. But I think this will make the first touch with libmongoc/libbson better.