Add documentation for Microcontroller (#603)

mesecons_microcontroller/doc/microcontroller/description.md

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+<!-- Generate `description.html` from this by using `$ md2html -o description.html description.md`. -->
+
+The Microcontroller is a semi-advanced programmable component with a persistent
+256 bit EEPROM memory.
+
+Warning: This device is largely considered deprecated and might contain bugs. It
+is recommended to use a Luacontroller instead.
+
+Detailed documentation can be found below:
+
+* The Microcontroller's code is executed whenever any of the following events
+  happens:
+  * The Microcontroller is programmed. In this case the EEPROM and ports are all
+    reset to `0` before.
+  * An incoming signal changes its state.
+  * An `after` event happens (see command `after` below).
+* There are 4 I/O ports (ABCD) and 256 EEPROM bits (1 to 256).
+* The code consists of a sequence of commands.
+* Everything after `:` is a comment.
+* Strings are enclosed in `"`s.
+* Spaces and tabs outside of strings are ignored.
+* Basic command syntax:
+  ```
+      command_name`(`param1`,` param2`,` ...`)`
+  ```
+* Commands:
+  * `if(condition) commands [> else_commands];`:
+    Evaluates the given condition and takes the corresponding branch.
+    The else branch is optional (as indicated by the `[` and `]`). The `>` is part
+    of the syntax and indicates the start of the else branch. The `;` marks the
+    end of the if command.
+  * `on(port1, port2, ...)`:
+    Sets the given ports to `1`.
+  * `off(port1, port2, ...)`:
+    Sets the given ports to `0`.
+  * `print("string" or codition, ...)`:
+    Evaluates the conditions and prints the concatenation of all params to stdout
+    (only useful in singleplayer).
+  * `after(time, "more commands")`:
+    Executes the commands in the string after the given time in seconds.
+    There can only be one waiting `after` event at once.
+    Warning: This is not reliable, ie. `minetest.after` is used.
+  * `sbi(port_or_eeprom, condition)`:
+    Evaluates the condition and sets the port or EEPROM bit to the resulting value.
+    Note: EEPROM indices don't use `#` here, ie. it's `sbi(1, #2)`, not `sbi(#1, #2)`.
+* Conditions (sorted by descending precedence; they are all evaluated from left
+  to right):
+  * `0`, `1`: constant
+  * `A`, ..., `D`: value of a port. Takes writes that already happened during the
+    current execution into account.
+  * `#1`, ..., `#256`: value of an EEPROM bit. Takes writes that already happened
+    during the current execution into account.
+  * `!condition`: negation (can only be applied once, ie. not `!!1`)
+  * `condition1 = condition2`: XNOR (equality)
+  * `condition1 op condition2` where `op` is one of:
+    * `&`: AND
+    * `|`: OR
+    * `~`: XOR (inequality)
+  * Note: Explicit precedence using parentheses is not supported.