[mesecons] Add NOR and OR gates, and documentation

mods/mesecons/mesecons_gates/doc/and/description.html

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+And gates power their output if both inputs (from left and right) are powered.

mods/mesecons/mesecons_gates/doc/diode/description.html

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+Mesecon diodes, just like real ones, only transfer power (signals) in one direction only.

mods/mesecons/mesecons_gates/doc/nand/description.html

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+Nand gates do not power their output if both inputs (from left and right) are powered, but power it in every other case.

mods/mesecons/mesecons_gates/doc/nor/description.html

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+Nor gates only power their output if none of their two inputs is powered. They are basically or gates with a not gate at their output.

mods/mesecons/mesecons_gates/doc/not/description.html

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+Not gates invert signals, just like a mesecon torch does, but faster. The input is at the opposite side of the output.

mods/mesecons/mesecons_gates/doc/or/description.html

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+Or gates power their output if either of their inputs (or both) are powered. You could basically get the same behaviour with two diodes, but or gates save some space.

mods/mesecons/mesecons_gates/doc/xor/description.html

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+XOR gates power their output if only one input is powered, they're off if either both or none of the inputs is powered.