[docs] RV32IM Extension specs

[manh9203]

• Added more detailed code comments for the circuit implementation
• Removed unnecessary constraints
• Added a README describing the statements each circuit must prove. Although it doesn’t prove circuit soundness by itself, it outlines the goal to be met when the circuit is modified.

[jonathanpwang]

@manh9203 can you separate this PR so that this one contains only doc/specs and no circuit changes?
It's best that all circuit changes are done in a standalone PR so it's easier to review and see the diffs.

[jonathanpwang]

LGTM, both markdown and code comments help a lot!

Some comments on wording improvements. In the core chip descriptions, please also include the assumptions on the input (mostly whether things should have been range checked already).

[github-actions]

group	app.proof_time_ms	app.cycles	app.cells_used	leaf.proof_time_ms	leaf.cycles	leaf.cells_used
verify_fibair	(+24 [+1.7%]) 1,396	147,020	8,194,664	-	-	-
fibonacci_program	(+13 [+0.3%]) 4,960	1,500,096	51,485,167	-	-	-
regex_program	13,977	4,140,164	167,389,450	-	-	-
ecrecover_program	(-35 [-1.4%]) 2,487	295,181	15,586,346	-	-	-

Commit: 3b42a87

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