docs: Add set.mitm documentation

docs/modules/set_mitm.md

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+# set_mitm
+set_mitm is a sysmodule that enables intercepting requests to the system settings service.
+
+## Atmosphère Extensions
+
+set_mitm intercepts the `GetFirmwareVersion` command, if the requester is `qlaunch` or `maintenance`.\
+It modifies the `display_version` field of the returned system version, causing the version to display\
+in settings as `#.#.# (AMS #.#.#)`. This allows users to easily verify what version of Atmosphère they are running.
+
+set_mitm also intercepts the `GetSettingsItemValueSize` and `GetSettingsItemValue` commands for all requesters.\
+It does so in order to enable user configuration of system settings, which are parsed from `atmosphere/system_settings.ini` on boot.\
+The format for settings is described below.
+
+### Atmosphère Settings Format
+
+Settings are parsed from the `atmosphere/system_settings.ini` file during the boot process. This file is a normal ini file,\
+with some specific interpretations.
+
+The standard representation of a system setting's identifier takes the form `name!key`. This is represented within\
+`system_settings.ini` as a section `name`, with an entry `key`. For example:
+
+```
+[name]
+key = ...
+```
+
+System settings can have variable types (strings, integral values, byte arrays, etc). To accommodate this, `system_settings.ini`\
+must store values as a `type_identifier!value_store` pair. A number of different types are supported, with identifiers detailed below.\
+Please note that a malformed value string will cause a fatal error to occur on boot. A full example of a custom setting is given below\
+(setting `eupld!upload_enabled = 0`), for posterity:
+
+```
+[eupld]
+upload_enabled = u8!0x0
+```
+
+### Supported Types
+
+* Strings
+ * Type identifiers: `str`, `string`
+ * The value string is used directly as the setting, with null terminator appended.
+* Integral types
+ * Type identifiers: `u8`, `u16`, `u32`, `u64`
+ * The value string is parsed via a call to `strtoul(value, NULL, 0)`.
+ * Setting bitwidth is determined by the identifier (8 for 1 byte, 16 for 2 bytes, and so on).
+* Raw bytes
+ * Type identifiers: `hex`, `bytes`
+ * The value string is parsed as a hexadecimal string.
+ * The value string must be of even length, or a fatal error will be thrown on parse.
+
+### Atmosphère Custom Settings
+
+At present, Atmosphère implements no custom settings. However, this is subject to change in the future, and any\
+custom settings will be documented here as they are added.