Progress docs

[willmcgugan]

Refinement to docs re new progress bar feature, and new examples.

[codecov-commenter]

Codecov Report

Merging #2141 (d852cd1) into master (7105781) will decrease coverage by 0.39%.
The diff coverage is 85.77%.

@@            Coverage Diff             @@
##           master    #2141      +/-   ##
==========================================
- Coverage   99.46%   99.07%   -0.40%     
==========================================
  Files          72       72              
  Lines        7339     7554     +215     
==========================================
+ Hits         7300     7484     +184     
- Misses         39       70      +31     

Flag	Coverage Δ
unittests	99.07% <85.77%> (-0.40%)	⬇️

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Impacted Files	Coverage Δ
rich/__main__.py	100.00% <ø> (ø)
rich/layout.py	100.00% <ø> (ø)
rich/progress.py	93.51% <77.69%> (-4.92%)	⬇️
rich/console.py	99.21% <100.00%> (+0.06%)	⬆️
rich/terminal_theme.py	100.00% <100.00%> (ø)
rich/traceback.py	99.55% <100.00%> (+<0.01%)	⬆️

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