Add a goldmark parser extension for first class sections

[iwahbe]

Adds section as a first class node kind in goldmark.

The test shows how sections are useful for manipulating the AST, since they encapsulate a header and it's children.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 57.37%. Comparing base (30a3a41) to head (f0483ae).

Files	Patch %	Lines
pkg/tfgen/parse/section/section.go	90.90%	4 Missing ⚠️
pkg/tfgen/parse/extension.go	89.28%	2 Missing and 1 partial ⚠️
pkg/tfgen/installation_docs.go	87.50%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2306      +/-   ##
==========================================
+ Coverage   56.83%   57.37%   +0.53%     
==========================================
  Files         362      364       +2     
  Lines       49886    49445     -441     
==========================================
+ Hits        28354    28368      +14     
+ Misses      19987    19530     -457     
- Partials     1545     1547       +2     

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[guineveresaenger]

This allows us to clean up the logic for skipSectionByHeader.