chore(book): cleanup examples

[jonathanpwang]

We have dedicated examples folder for book example code. Updates some links and also uses mdbook anchors to directly reference the code.

closes INT-2964

[github-actions]

group	app.proof_time_ms	app.cycles	app.cells_used	leaf.proof_time_ms	leaf.cycles	leaf.cells_used
verify_fibair	(+18 [+1.2%]) 1,510	194,687	8,011,682	-	-	-
fibonacci_program	(+38 [+0.6%]) 6,549	1,500,137	51,505,102	-	-	-
regex_program	(-184 [-0.9%]) 19,649	4,190,904	165,028,173	-	-	-
ecrecover_program	(-3 [-0.1%]) 2,752	290,016	15,230,037	-	-	-

Commit: 2a92987

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