Improve conda installation instructions

[mefuller]

Motivation or Problem

Writing the documentation to only reference Anaconda is inflexible and bloated as compared with utilizing Miniconda or Conda available in system repositories. This is of particular relevance of storage-limited systems, such as shared high-performance resources where RMG is often installed (which also frequently run RHEL or another Enterpise Linux which includes conda in the repositories, easing administrative burden).

Description of Changes

• Use repositories over Ananconda for proper locations, updates
• switch to Miniconda to reduce download and installation size by 10x
• replace deprecated source with conda in activating environments

Testing

Environments created and RMG-Py, RMG-database installed and tested on Fedora and EL systems