A bunch of small dev-oriented documentation updates

[drewoldag]

Addressing requests in issue #51

Adding note about using mamba as a faster alternative to conda.
Mamba not conda so it's simpler
rail --help,
Explain why the instructions are what they are (pip vs. conda)
Explain the command-line tool (purpose, what this gives you) rather than just jumping into the commands
Change pip install to pip install -e .

Code Quality

• My code follows the code style of this project
• I have written unit tests or justified all instances of #pragma: no cover; in the case of a bugfix, a new test that breaks as a result of the bug has been added
• My code contains relevant comments and necessary documentation for future maintainers; the change is reflected in applicable demos/tutorials (with output cleared!) and added/updated docstrings use the NumPy docstring format
• Any breaking changes, described above, are accompanied by backwards compatibility and deprecation warnings

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e015e13) 100.00% compared to head (880653b) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #120   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines            1         1           
=========================================
  Hits             1         1           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[OliviaLynn]

This looks great!