[RFC] TVM Website Style Update

To support the TVM user and developer community, this RFC proposes a style update to the TVM website to improve its appearance and usability. The PR for this proposal is up at https://github.com/apache/incubator-tvm-site/pull/15

This is largely a style update, with most of the content untouched. You can run a version of the site by cloning the repository with the patchset, and running ./serve_local.sh to launch a local copy of the site with the updated content.

This RFC is seeking lazy consensus from the TVM community, with feedback here and on community reviews on the PR.

Thanks! Chris