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

• [x] My code follows the code style of this project
• [x] 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
• [x] 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
• [x] Any breaking changes, described above, are accompanied by backwards compatibility and deprecation warnings

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

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