docs: expand astronomical argument formalism for #354

[tsutterley]

docs: add example for plotting tidal amplitude spectra
docs: split background into separate pages
docs: add project info

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 70.04%. Comparing base (69f841b) to head (fc67486).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pyTMD/predict.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #382      +/-   ##
==========================================
- Coverage   70.05%   70.04%   -0.02%     
==========================================
  Files          42       42              
  Lines       10740    10743       +3     
  Branches     1159     1159              
==========================================
+ Hits         7524     7525       +1     
- Misses       2753     2755       +2     
  Partials      463      463              

Flag	Coverage Δ
unittests	70.04% <33.33%> (-0.02%)	⬇️

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