Add developers doc; accordingly rename abstract supertypes

[Datseris]

Closes #71

This adds a new page "Developers docs" that outlines how to contribute new pipelines to the software.

Additionally, I renamed the abstract types, and the core documentation strings, to talk about only two things: "changes" and "statistically significant changes" the latter being equivalent with "transitions".

I wanted to move away from referring to "indicators" all the time, as we have already discussed that sometimes an indicator is not used, only a change metric is used.

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (b1d3387) 36.42% compared to head (9e1706c) 45.22%.

Files	Patch %	Lines
src/significance/basic_stat_significance.jl	0.00%	3 Missing ⚠️
src/analysis/segmented_window.jl	0.00%	1 Missing ⚠️
src/significance/api_significance.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
+ Coverage   36.42%   45.22%   +8.79%     
==========================================
  Files          18       17       -1     
  Lines         442      356      -86     
==========================================
  Hits          161      161              
+ Misses        281      195      -86     

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