Prepare v0.4 release

[mkelley]

• Updated change log
• Verified and updated URLs as needed
• Updated sub-module status page.
• Set a release date.

[codecov[bot]]

Codecov Report

Merging #375 (343786a) into main (39451c6) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #375   +/-   ##
=======================================
  Coverage   76.80%   76.80%           
=======================================
  Files          78       78           
  Lines        6984     6984           
=======================================
  Hits         5364     5364           
  Misses       1620     1620           

Impacted Files	Coverage Δ
sbpy/data/obs.py	24.19% <ø> (ø)
sbpy/data/orbit.py	30.57% <ø> (ø)

[mkelley]

@jianyangli This is ready for review. I set the release date to today, but that's not important. You can review whenever you have an opportunity.