doc: explanation of how libraries depend on one another (#3469)

Check out
https://canonical-mir--3469.com.readthedocs.build/3469/explanation/libraries/

doc/sphinx/explanation/index.md

@@ -2,6 +2,7 @@
 These pages provide additional detail about a number of aspects related to using Mir.
 
 - [Architecture](architecture): an overview of Mir's architecture for contributors
+- [Libraries](libraries): an overview of Mir's libraries and how they depend on one another
 - [Graphics support](mir-graphics-support): what's required to run Mir compositors
 - [What is Wayland anyway?](ok-so-what-is-this-wayland-thing-anyway): so what is it?
 - [Windowing paradigms](window-positions-under-wayland): how are windows managed?
@@ -11,6 +12,7 @@ These pages provide additional detail about a number of aspects related to using
 :hidden:
 
 architecture
+libraries
 mir-graphics-support
 ok-so-what-is-this-wayland-thing-anyway
 window-positions-under-wayland

doc/sphinx/explanation/libraries.md

@@ -0,0 +1,52 @@
+# Libraries
+The Mir project is a collection of C++ libraries for writing Wayland
+compositors. This document describes what those libraries are and how they
+depend on one another.
+
+## Public Libraries
+The following libraries are intended for published for public consumption:
+
+- `miral`
+- `mircommon`
+- `mircore`
+- `miroil`
+- `mirplatform`
+- `mirserver`
+- `mirserverlttng`
+- `mirwayland`
+
+When you build the project, these libraries are found in
+`<build_directory>/lib`.
+
+Additionally, Mir publishes libraries that add support for different platforms.
+These can be found in `<build_directory>/lib/server-modules`. These libraries
+are loaded at runtime by Mir and provide access to the underlying graphics and
+input hardware of that platform. The libraries that Mir provides are:
+
+- `graphics-gbm-kms`
+- `graphics-eglstream-kms`
+- `graphics-wayland`
+- `graphics-dummy`
+- `server-x11`
+- `server-virtual`
+- `renderer-egl-generic`
+- `input-evdev`
+- `input-stub`
+
+## Dependency Graph
+In the following diagram, each arrow denotes that the library at the start of
+the arrow depends on the library at the end of the arrow.
+
+```{mermaid} libraries.mmd
+```
+
+There are a few things of note in this diagram:
+
+- `miral` and `mircore` are intended to be the primary user-facing libraries
+- `miroil` is a special compatibility layer for the Lomiri project is not
+  intended for general consumption
+- `mirserver` depends on the most libraries as it is the brains of the operation
+- `mircommon` provide common functionalities for every other library in the
+  system
+- The input and graphics platforms (represented as `input_X` and `graphics_X` in
+  the diagram) are dynamically loaded by `mirserver` when the engine starts up.

doc/sphinx/explanation/libraries.mmd

@@ -0,0 +1,19 @@
+flowchart
+    miral ---> mirserver
+    miral ---> mircommon
+    miral ---> mircore
+    miroil ---> mirserver
+    miroil --> mircommon
+    mirserver --> mirplatform
+    mirserver --> mircommon
+    mirserver --> mirserverlttng
+    mirserver --> mircore
+    mirserver --->|dynamically loads|graphics_X
+    mirserver --->|dynamically loads|input_X
+    mirwayland --> mircore
+    graphics_X --> mirplatform
+    graphics_X --> mircommon
+    graphics_X --> mircore
+    input_X --> mirplatform
+    input_X --> mircommon
+    input_X --> mircore