Clarify the optional Anaconda installation step in docs

[e-kayrakli]

This PR modifies the Linux installation guide to clarify that Anaconda installation is a step separate from Arkouda installation and should be done if Anaconda is not installed already.

I haven't figured out how to build docs locally and check the rendered output, so, creating this as a draft PR for the time being.

[stress-tess]

I haven't figured out how to build docs locally and check the rendered output, so, creating this as a draft PR for the time being.

@e-kayrakli, you might've already figured this out but just in case you haven't. Running the following commands should do the trick (or at least give us more info):

make doc-clean
make doc
open docs/index.html

There is some documentation for this, but it's not the best and kinda hidden away in the build section:
https://bears-r-us.github.io/arkouda/setup/BUILD.html#building-the-arkouda-documentation