doc: services: add structure/categorization to services index

The very flat and unorganized list of services was really difficult to
navigate, and not really helping people _discover_ services if they
didn't already know what they were looking for.
This tries to address that by adding some grouping.

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

doc/services/index.rst

@@ -3,42 +3,192 @@
 OS Services
 ###########
 
+Zephyr provides a comprehensive set of subsystems and associated services that applications can
+leverage.
+
+.. will move the CSS below to custom.css when done with experimenting :)
+
+.. raw:: html
+
+    <style>
+      .sd-card {
+        background-color: var(--navbar-background-color);
+        border-radius: 0.75rem !important;
+        border: 1px solid transparent;
+        transition: all 0.15s ease;
+        box-shadow: none !important;
+      }
+      .sd-card:hover {
+        transform: translateY(-3px);
+        box-shadow: 0 6px 12px rgba(0, 0, 0, 0.25) !important;
+        border-color: rgba(255, 255, 255, 0.05);
+      }
+      .sd-card-title {
+        font-size: 1.125rem !important;
+        font-weight: bold !important;
+        margin-bottom: 0.25rem !important;
+        color: #fff !important;
+      }
+      .sd-card-text {
+        color: var(--navbar-level-1-color) !important;
+        font-weight: 300 !important;
+        font-size: 0.9rem !important;
+        margin: 0 !important;
+        opacity: 0.85;
+      }
+    </style>
+
+.. grid:: 1 1 2 2
+    :gutter: 2
+
+    .. grid-item-card:: :material-twotone:`memory;36px` System
+        :link: system_services
+        :link-type: ref
+
+        Memory management, low-level infrastructure, and other utility APIs.
+
+    .. grid-item-card:: :material-twotone:`sync;36px` Data Processing
+        :link: data_processing_services
+        :link-type: ref
+
+        Data transformation, encoding, and validation APIs.
+
+    .. grid-item-card:: :material-twotone:`bug_report;36px` Debugging & Observability
+        :link: debugging_observability_services
+        :link-type: ref
+
+        Logging, tracing, shell, and other debugging and observability tools.
+
+    .. grid-item-card:: :material-twotone:`battery_charging_30;36px` Power Management
+        :link: power_management_services
+        :link-type: ref
+
+        Energy consumption, performance scaling, and resource lifecycle control.
+
+    .. grid-item-card:: :material-twotone:`save;36px` Storage
+        :link: storage_reference
+        :link-type: ref
+
+        Flash access, file systems, key-value stores, and persistent data.
+
+    .. grid-item-card:: :material-twotone:`mail;36px` Communication
+        :link: communication_services
+        :link-type: ref
+
+        Inter-thread, inter-processor, and external communication mechanisms.
+
+    .. grid-item-card:: :material-twotone:`security;36px` Security
+        :link: security_services
+        :link-type: ref
+
+        Cryptographic primitives and trusted execution environments.
+
+    .. grid-item-card:: :material-twotone:`widgets;36px` Application Services
+        :link: application_framework_services
+        :link-type: ref
+
+        High-level frameworks and libraries for application development.
+
+.. toctree::
+   :maxdepth: 2
+   :hidden:
+
+   storage/index.rst
+
+.. _system_services:
+
+System
+******
+
 .. toctree::
    :maxdepth: 1
 
+   mem_mgmt/index.rst
+   net_buf/index.rst
+   rtio/index.rst
+   notify.rst
    binary_descriptors/index.rst
-   console.rst
-   cpu_freq/index.rst
-   cpu_load/index.rst
+   portability/index.rst
+   task_wdt/index.rst
+
+.. _data_processing_services:
+
+Data Processing
+***************
+
+.. toctree::
+   :maxdepth: 1
+
    crc/index.rst
-   crypto/index
-   debugging/index.rst
-   device_mgmt/index
    dsp/index.rst
-   formatted_output.rst
-   input/index.rst
-   instrumentation/index.rst
-   ipc/index.rst
-   llext/index.rst
+   serialization/index.rst
+   misc.rst
+
+.. _debugging_observability_services:
+
+Debugging & Observability
+*************************
+
+.. toctree::
+   :maxdepth: 1
+
    logging/index.rst
    tracing/index.rst
-   resource_management/index.rst
-   mem_mgmt/index.rst
-   net_buf/index.rst
-   modem/index.rst
-   notify.rst
-   pm/index.rst
-   portability/index.rst
-   poweroff.rst
+   debugging/index.rst
    profiling/index.rst
+   instrumentation/index.rst
+   cpu_load/index.rst
    shell/index.rst
-   serialization/index.rst
+
+.. _power_management_services:
+
+Power Management
+****************
+
+.. toctree::
+   :maxdepth: 1
+
+   pm/index.rst
+   cpu_freq/index.rst
+   poweroff.rst
+   resource_management/index.rst
+
+.. _communication_services:
+
+Communication
+*************
+
+.. toctree::
+   :maxdepth: 1
+
+   zbus/index.rst
+   ipc/index.rst
+   modem/index.rst
+   virtualization/index.rst
+   console.rst
+   input/index.rst
+   formatted_output.rst
+
+.. _security_services:
+
+Security
+********
+
+.. toctree::
+   :maxdepth: 1
+
+   crypto/index
+   tfm/index
+
+.. _application_framework_services:
+
+Application Services
+********************
+
+.. toctree::
+   :maxdepth: 1
+
    smf/index.rst
-   storage/index.rst
    sensing/index.rst
-   task_wdt/index.rst
-   tfm/index
-   virtualization/index.rst
-   rtio/index.rst
-   zbus/index.rst
-   misc.rst
+   device_mgmt/index
+   llext/index.rst