Add docs with examples to Node classes

[robinst]

Fixes #356.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.94%. Comparing base (40f1fd5) to head (245c345).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #370   +/-   ##
=========================================
  Coverage     94.94%   94.94%           
  Complexity      252      252           
=========================================
  Files           138      138           
  Lines          4471     4471           
  Branches        646      646           
=========================================
  Hits           4245     4245           
  Misses          122      122           
  Partials        104      104           

Files with missing lines	Coverage Δ
.../src/main/java/org/commonmark/node/BlockQuote.java	100.00% <ø> (ø)
.../src/main/java/org/commonmark/node/BulletList.java	66.66% <ø> (ø)
...onmark/src/main/java/org/commonmark/node/Code.java	100.00% <ø> (ø)
...src/main/java/org/commonmark/node/CustomBlock.java	100.00% <ø> (ø)
.../src/main/java/org/commonmark/node/CustomNode.java	100.00% <ø> (ø)
...rk/src/main/java/org/commonmark/node/Document.java	100.00% <ø> (ø)
...rk/src/main/java/org/commonmark/node/Emphasis.java	81.81% <ø> (ø)
...main/java/org/commonmark/node/FencedCodeBlock.java	64.86% <ø> (ø)
...c/main/java/org/commonmark/node/HardLineBreak.java	100.00% <ø> (ø)
...ark/src/main/java/org/commonmark/node/Heading.java	100.00% <ø> (ø)
... and 12 more

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