[FEA] URL for library docs should be in docs.rapids.ai/libname or libname.rapids.ai , rather than docs.rapids.ai/api/libname

[shwina]

There are two reasons for this request:

• I can never remember the URL for docs off the top of my head because it's so complicated. Pandas for example is just pandas.pydata.org. From rapids.ai, it takes at least 3 clicks to get to the docs for the individual libraries, and that is if you know exactly what you are doing.
• The documentation for cudf, cuml, etc., consists of more than just the API reference. They also contain concepts, user guides, etc. The URL is thus a bit misleading.