Fix docs tox environment

[alexanderronquillo]

The API documentation was empty because autosummary couldn't import the source modules. I fixed this by installing merlin-core in the docs tox environment.

Note: When I generate the docs now, I see some import warnings for other packages. However, I opted to ignore those for now because I was having trouble installing one of them (cudf) into the environment, and it didn't prevent the docs from generating.

I've also included a couple of small README changes:

• Fixed a typo
• Added a note about using Python 3.9 to generate documentation

[mikemckiernan]

Huge thanks for doing the sleuthing. Up to you whether you want to change how the core library is installed or not.

[oliverholworthy]

I suggested this pip install in another comment, however I think it may not be strictly necessary, since tox installs the package dependencies as part of it's environment setup. And merlin-core is specified in the package install requires.

I tried running this docs and docs-multi locally without this and both seem to produce the API docs with content, so might be some other reason they didn't populate

[alexanderronquillo]

hmm. ok, yeah then I'm stumped on what the problem would have been.