Document use of 'Draft' status when reviewing PRs

[jsiirola]

Fixes # .

Summary/Motivation:

As discussed in the Dev Call, this PR proposes that we use Draft status to indicate that a PR is waiting on an author to provide input (and not on the dev team to provide feedback).

Changes proposed in this PR:

• Document use of "Draft" status to indicate "waiting on PR author"
• Update install instructions to use pip and not setup.py
• Update the layout / formatting of the contribution guide page
• Cleans up trailing whitespace

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.93%. Comparing base (fcb8bdf) to head (30aef12).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3918   +/-   ##
=======================================
  Coverage   89.93%   89.93%           
=======================================
  Files         902      902           
  Lines      106457   106457           
=======================================
  Hits        95745    95745           
  Misses      10712    10712           

Flag	Coverage Δ
builders	29.17% <ø> (-0.01%)	⬇️
default	86.24% <ø> (?)
expensive	35.64% <ø> (?)
linux	87.39% <ø> (-2.04%)	⬇️
linux_other	87.39% <ø> (-0.01%)	⬇️
oldsolvers	28.11% <ø> (ø)
osx	82.72% <ø> (ø)
win	85.81% <ø> (ø)
win_other	85.81% <ø> (ø)

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