Documentation fixes

[gadomski]

• Removes a duplicate short option from the merge cli command. This could break downstream user's workflows but that's unlikely, and I can't find anything saying what happens when there are duplicate short options in click so we'll say that was undefined behavior anyways.
• Fixes the sentinel2 path in the readthedocs config that was breaking readthedocs builds.
• Clean up sphinx warnings by adding the tutorials to the TOC and fixing some bad links.

[codecov-commenter]

Codecov Report

Merging #93 (f61d2dd) into master (ae72934) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #93   +/-   ##
=======================================
  Coverage   85.25%   85.25%           
=======================================
  Files          81       81           
  Lines        2421     2421           
=======================================
  Hits         2064     2064           
  Misses        357      357           

Impacted Files	Coverage Δ
stactools_cli/stactools/cli/commands/merge.py	87.50% <100.00%> (ø)

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