Resolve "Describe supported OSes"

Closes #1055 (closed)

@tomek Mrugalski I created a platforms.md file, and deleted the same info in the intro.rst file. However, I think that the platforms information should still be included in the ARM (and Suzanne thought she remembered that it doesn't have to be a .rst file, it can also just be a .md file and be included). I am not sure how to address a link to the file, and I am afraid of messing with the build file for the docs. All of the files are in the same directory today main_sources=$(rst_arm_sources) conf.py $(static_sources)

Similarly, there are one or two links in the platforms.md text (e.g. to build requirements) that I think need to be updated, but again, I am not sure how to reference the location since it is in a different directory now from platforms.md. I put platforms.md at the top level, as it is in the BIND repo, so it is easily visible to someone visiting the repo. also see :ref:build-requirements` - is this going to work if platforms.md is in a different place in the hierarchy now?