Refactor docs, CI

[tony]

Reminder (same as tmux-python/libtmux#295): must keep setup.py and requirements - at least for base.txt.

Linux / BSD packaging support for poetry built packages is unknown

Move CI from Travis to Github actions

[codecov]

Codecov Report

Merging #623 into master will decrease coverage by 1.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #623      +/-   ##
==========================================
- Coverage   78.32%   77.08%   -1.24%     
==========================================
  Files           5        6       +1     
  Lines         798      829      +31     
  Branches      232      238       +6     
==========================================
+ Hits          625      639      +14     
- Misses        115      131      +16     
- Partials       58       59       +1     

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)
tmuxp/cli.py	66.00% <ø> (+1.13%)	⬆️
tmuxp/workspacebuilder.py	88.48% <0.00%> (-0.61%)	⬇️

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