Add a first example ingestion script to the docs

[tomalrussell]

This adds the example discussed in #271 to the getting started docs, aiming to provide a first example script that uses as little as possible of the terracotta API and not much core Python or other libraries either! Hopefully a good starting point for more novice users.

[codecov]

Codecov Report

Merging #277 (6b8b85a) into main (52f5881) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #277      +/-   ##
==========================================
- Coverage   98.82%   98.77%   -0.05%     
==========================================
  Files          49       49              
  Lines        2211     2211              
  Branches      338      338              
==========================================
- Hits         2185     2184       -1     
- Misses         15       17       +2     
+ Partials       11       10       -1     

Impacted Files	Coverage Δ
terracotta/scripts/optimize_rasters.py	95.94% <0.00%> (-0.68%)	⬇️

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[dionhaefner]

Fantastic, thanks!