[DOCS] Sphinx docs warning fixes

[tqchen]

This issue happens when the docker update status for ci-gpu to the lastest(the main change is is mkl-ml), and it also triggers a latest sphinx update.

Currently log for the sphinx pre-check warnings due to the version upgrade http://ci.tvm.ai:8080/job/temp-ci-docker-staging/job/ci-stage/29/execution/node/205/log/ need to fix them before the we upgrade the image.

Actions to be taken

• Migrate the markdown to rst
  • cc @tmoreau89 for docs/vta/install.md
  • cc @kazum for docs/deploy/aws_fpga.md, aocl.md
• fix the remaining warnings

[tqchen]

All the docs change are landed by now