docs: Bluetooth: Add sections on GTBS and GMCS

The API reference for media control can be found in

:ref:`Bluetooth Media Control <bluetooth_media>`.

Generic TBS and Generic MCS

~~~~~~~~~~~~~~~~~~~~~~~~~~~

Both the Telephone Bearer Service (TBS) used by CCP and the Media Control Service (MCS) used by MCP

have the concept of generic instances of the services called Generic TBS (GTBS) and

Generic MCS (GMCS).

While these share a common name prefix, the behavior of these two may be significantly different.

Generic TBS

^^^^^^^^^^^

The TBS spec defines GTBS as

   GTBS provides a single point of access and exposes a representation of its internal telephone

   bearers into a single telephone bearer.

   This service provides telephone status and control of the device as a single unit with a

   single set of characteristics.

   It is left up to the implementation to determine what telephone bearer a characteristic of

   GTBS represents at any time.

   There is no specified manner of representing a characteristic from each individual TBS that

   resides on the device to the same characteristic of the GTBS.

   For example, if there is more than one TBS on a device and each has a unique telephone bearer

   name (e.g., Name1 and Name2),

   the way the GTBS represents the telephone bearer name is left up to the implementation.

   GTBS is suited for clients that do not need to access or control all the

   information available on specific telephone bearers.

This means that a GTBS instance represents one or more telephone bearers.

A telephone bearer could be any application on a device that can handle (telephone) calls,

such as the default Call application on a smartphone,

but also other applications such as Signal, Discord, Teams, Slack, etc.

GTBS may be standalone (i.e.the device only has a GTBS instance without any TBS instances),

and the behavior of the GTBS is mostly left up to the implementation.

In Zephyr the implementation of GBTS is that it contains some generic information,

such as the provider name which is defined to  simply be "Generic TBS",

but the majority of the information in the GTBS instance in Zephyr has been implemented to be a

union of the data of the other bearers.

For example if you have a bearer for regular phone calls and

Teams and have an active call in both bearers,

then each of those bearers will report a single call,

but the GTBS instance will report 2 calls,

making it possible for a simple Call Control Client to control all calls from a single bearer.

Similarly the supported URIs for each bearer are also made into a union in GTBS, and when placing

a call using the GTBS the server will pick the most suited bearer depending on the URI.

For example calls with URI `tel` would go to the regular phone application,

and calls with the URI `skype` would go to the Teams application.

In conclusion the GTBS implementation in Zephyr is a union of the non-generic telephone bearers.

Generic MCS

^^^^^^^^^^^

The MCS spec defines GMCS as

   The GMCS provides status and control of media playback for the device as a single unit.

   An MCS instance describes and controls the media playback for a

   specific media player within the device.

   A device implements MCS instances to allow clients to access the

   separate internal media player entities.

and where the behavior of GMCS is defined as

   ... the behavior of MCS and GMCS is identical,

   and all the characteristics and the characteristics' behaviors are the same.

   The term “MCS” is used throughout the document.

   Unless otherwise specifically stated in this specification,

   the same meaning applies to GMCS as well.

This means that a GMCS instance works the same way as an MCS instance,

and it follows that GMCS

   controls the media playback for a specific media player within the device

A media player on a device could be anything that plays media,

such as a Spotify or Youtube application on a smartphone.

Thus if a device has multiple MCS instances,

then each of these control media for that specific application,

but the GMCS also controls media playback for a specific media player.

GMCS can thus be considered a pointer to a specific MCS instance,

and control either e.g. Spotify or Youtube, but not both.

The MCS spec does however provide an example of GMCS where a device can

   Implement a GMCS that provides status and control of media playback for the device as a whole.

Which may indicate that an implementation may use GMCS to represent all media players with GMCS and

not a specific media player as stated above. In the case where a device does not have any MCS

instances and only GMCS, then GMCS will point to a generic instance.

The Zephyr implementation of MCS and GMCS is incomplete,

and currently only supports instantiating a single instance that can either be an MCS or GMCS.

This means that the implementation is neither complete nor spec-compliant.

Difference between GTBS and GMCS

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The definitions and implementations of GTBS and GMCS as stated above are notably different.

GTBS works as a union between the other TBS instances (if any),

and GMCS works as a pointer to a specific MCS instance (if any).

This effectively means that a simple Call Control Client can control all calls just using GTBS,

but a Media Control Client may only be able to control a single player using GMCS.

Coordinated Sets

----------------