add markdown spec for `begin_tx` & `end_tx` gadgets

[DreamWuGit]

currently there is only python implementation for begin_tx & end_tx gadgets, I think it is better to add markdown spec as well like other auxiliary gadgets(CopyToMemory, CopyToLog etc.) , which is useful for better understanding and reading !

Glad to know your opinions!

[ed255]

You can add end_block to the list :P

[ChihChengLiang]

Adding markdown spec for begin_tx, end_tx, CopyToMemory, CopyToLog, and end_block sounds like a good idea to me!

[ed255]

Note that CopyToMemory and CopyToMemory execution state no longer exits (the became obsolete with the copy circuit).

[rrtoledo]

I added some comments/questions on the PR directly