docs: "Project structure" chapter expanded

docs/getting_started/installation_and_usage.md

@@ -4,9 +4,12 @@ This chapter provides all the necessary information to obtain and build the code
 It also describes how to build or distribute the library and generate its documentation.
 
 
-## Repository structure and dependencies
+## Project structure
 
-This repository contains three independent CMake-based projects:
+### Repository directory tree and dependencies
+
+The [GitHub repository](https://github.com/mpusz/mp-units) contains three independent CMake-based
+projects:
 
 - **_./src_**
 
@@ -55,8 +58,7 @@ This repository contains three independent CMake-based projects:
     To learn more about the rationale, please check our
     [FAQ](faq.md#why-dont-we-have-cmake-options-to-disable-building-of-tests-and-examples).
 
-
-## Modules
+### Modules
 
 The **mp-units** library provides the following C++ modules:
 
@@ -73,8 +75,81 @@ flowchart TD
 
 !!! note
 
-    C++ modules are provided within the package only when [`cxx_modules`](#cxx_modules) Conan
-    option is set to `True`.
+    C++ modules are provided within the package only when:
+
+    - [`cxx_modules`](#cxx_modules) Conan option is set to `True`,
+    - [`MP_UNITS_BUILD_CXX_MODULES`](#MP_UNITS_BUILD_CXX_MODULES) CMake option is set to `ON`.
+
+### Header files
+
+All of the project's header files can be found in the `mp-units/...` subdirectory.
+
+#### Core library
+
+- `mp-units/framework.h` contains the entire library's framework definitions,
+- `mp-units/concepts.h` exposes only the library's concepts for generic code needs,
+- `mp-units/format.h` provides text formatting support,
+- `mp-units/ostream.h` enables streaming of the library's objects to the text output,
+- `mp-units/math.h` provides overloads of common math functions for quantities,
+- `mp-units/random.h` provides C++ pseudo-random number generators for quantities,
+- `mp-units/compat_macros.h` provides macros for [wide compatibility](../users_guide/use_cases/wide_compatibility.md).
+
+??? info "More details"
+
+    More detailed header files can be found in subfolders which typically should not be
+    included by the end users:
+
+    - `mp-units/framework/...` provides all the public interfaces of the framework,
+    - `mp-units/bits/...` provides private implementation details only (no public definitions),
+    - `mp-units/ext/...` contains external dependencies that at some point in the future should
+      be replaced with C++ standard library facilities.
+
+#### Systems and associated utilities
+
+The systems definitions can be found in the `mp-units/systems/...` subdirectory:
+
+##### Systems of quantities
+
+- `mp-units/systems/isq.h` provides
+  [International System of Quantities (ISQ)](https://en.wikipedia.org/wiki/International_System_of_Quantities)
+  definitions,
+
+??? tip "Tip: Improving compile times"
+
+    `mp-units/systems/isq.h` might be expensive to compile in every translation unit. There are
+    some smaller, domain targeted files available for explicit inclusion in the
+    `mp-units/systems/isq/...` subdirectory.
+
+##### Systems of units
+
+- `mp-units/systems/si.h` provides
+  [International System of Units (SI)](https://en.wikipedia.org/wiki/International_System_of_Units)
+  definitions and associated math functions,
+- `mp-units/systems/angular.h` provides strong angular units and associated math functions,
+- `mp-units/systems/international.h` provides
+  [international yard and pound](https://en.wikipedia.org/wiki/International_yard_and_pound) units,
+- `mp-units/systems/imperial.h` includes `international.h` and extends it with
+  [imperial units](https://en.wikipedia.org/wiki/Imperial_units),
+- `mp-units/systems/usc.h` includes `international.h` and extends it with
+  [United States customary system of units](https://en.wikipedia.org/wiki/United_States_customary_units),
+- `mp-units/systems/cgs.h` provides
+  [centimetre-gram-second system of units](https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units),
+- `mp-units/systems/iau.h` provides
+  [astronomical system of units](https://en.wikipedia.org/wiki/Astronomical_system_of_units),
+- `mp-units/systems/hep.h` provides units used in
+  [high-energy physics](https://en.wikipedia.org/wiki/Particle_physics),
+- `mp-units/systems/typographic.h` provides units used in
+  [typography or typesetting](https://en.wikipedia.org/wiki/Typographic_unit),
+- `mp-units/systems/natural.h` provides an example implementation of
+  [natural units](https://en.wikipedia.org/wiki/Natural_units).
+
+??? tip "Tip: Improving compile times"
+
+    `mp-units/systems/si.h` might be expensive to compile in every translation unit.
+    There are some smaller files available for explicit inclusion in the
+    `mp-units/systems/si/...` subdirectory.
+
+    `mp-units/systems/si/unit_symbols.h` is the most expensive to include.
 
 
 ## Obtaining dependencies