Fix documentation issues

[nuclearGoblin]

Not meant to resolve all github issues with documentation - the primary purpose was to resolve #53 and the documentation issue discussed in #48, but I included some other improvements as well.

• Resolve #53 by changing lifetime references of units to those used in the code and levelfiles (femtoseconds, not attoseconds).
• Resolve #34 by creating a "Workflow" section in the CONTRIBUTING.
• Clarify syntax for verbosity flags (see #48).
• Add badge for arXiv (see #37).
• Improved CONTRIBUTING organization.
  • Created pull_request_template.md as a separate file.
  • Removed templates from CONTRIBUTING - using GitHub's management of templates instead.

[nuclearGoblin]

Requesting review mostly on the changes to CONTRIBUTING, since I restructured it. Are you okay with moving the code of conduct and all the templates out of the contributing file? Also, I mostly copied what you wrote on the "Workflow" section, but I did make some changes to generalize it - are those still the correct steps?

[nuclearGoblin]

@villaa Okay, I think I addressed everything we talked about - the only downside is that adding the sphinx requirements to the conda environment seems to have drastically increased the environment build time, depsite adding version numbers for all of the requirements. That said, it's still short enough that the tests pass.

[nuclearGoblin]

Settling on - since most users aren't building the docs, as they're available online already, the "default" provided environment shouldn't include those dependencies at a cost to the user's time. So, I'm keeping that version of the environment file as a separate yaml (nrCascadeSim_withDocs.yml), and switching the main one back so that anyone who doesn't intend to build the docs can still build the environment in a reasonable amount of time.