Additional guidelines for how to use Conda focusing on tasks

[YankeMao]

Write the instructions for four major types of tasks of conda, which are Managing conda, Managing environments, Managing channels, and Managing packages. Introduced the guidelines of these four tasks and the commonly used commands for these tasks. This PR is related to the issue: Development Process: Introduction for the Conda #484

[YankeMao]

Hi Yanke, I think your pr looks great. Here are a few suggestions for making it more clear.

1. Add a few sentences on the high level idea of managing conda in the beginning of the file
2. You can also consider adding a description for each sub-section like managing environments
3. Also, I think it would be very helpful to add a table of contents in the begining

Hi Emma, thank you for your reviewing! I did two major editions to deal with the issue you raised:

1. Split the file into four separate files where each file contains one type of task.
2. Add a high-level description at the beginning of each file.

With these changes, now we have high-level descriptions for each type of task. Meanwhile, as we split the file, now each file only had one type of task, which made the file length much shorter than before. Therefore, the table of contents at the beginning is not necessary in this case.