Add Platform overview post

This needs review (and likely edits) before merge. @avsm should do this and rebuild site after merging.

Should also check that images are still appropriate and that they display as expected. I wasn't sure about URL structure.
Perhaps:

"While OPAM allows users to compile programs from source, it is important to support downloading pre-build binary executables. This is especially important for slower platforms such as the Raspberry Pi."