docs: rework the driver-api top-level page

Add subsections in an attempt to bring a bit order to this page; also sort
most subsections into alphabetical order.  With luck all this will help to
prevent merge conflicts on this page due to everybody adding entries at the
end.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/87plxtm2oo.fsf@meer.lwn.net

Documentation/driver-api/index.rst

@@ -9,110 +9,141 @@ of device drivers.  This document is an only somewhat organized collection
 of some of those interfaces — it will hopefully get better over time!  The
 available subsections can be seen below.
 
+
+General information for driver authors
+======================================
+
+This section contains documentation that should, at some point or other, be
+of interest to most developers working on device drivers.
+
 .. toctree::
-   :caption: Table of contents
-   :maxdepth: 2
+   :maxdepth: 1
 
-   driver-model/index
    basics
+   driver-model/index
+   device_link
    infrastructure
    ioctl
-   early-userspace/index
    pm/index
-   clk
+
+Useful support libraries
+========================
+
+This section contains documentation that should, at some point or other, be
+of interest to most developers working on device drivers.
+
+.. toctree::
+   :maxdepth: 1
+
+   early-userspace/index
+   connector
    device-io
+   devfreq
    dma-buf
-   device_link
    component
-   message-based
-   infiniband
-   aperture
-   frame-buffer
-   regulator
-   reset
-   iio/index
-   input
-   usb/index
-   firewire
-   pci/index
+   io-mapping
+   io_ordering
+   uio-howto
+   vfio-mediated-device
+   vfio
+   vfio-pci-device-specific-driver-acceptance
+
+Bus-level documentation
+=======================
+
+.. toctree::
+   :maxdepth: 1
+
+   auxiliary_bus
    cxl/index
-   spi
-   i2c
-   ipmb
-   ipmi
+   eisa
+   firewire
    i3c/index
-   interconnect
-   devfreq
-   hsi
-   edac
-   scsi
-   libata
-   target
-   mailbox
-   mtdnand
-   miscellaneous
-   mei/index
-   mtd/index
-   mmc/index
-   nvdimm/index
-   w1
+   isa
+   men-chameleon-bus
+   pci/index
    rapidio/index
-   s390-drivers
+   slimbus
+   usb/index
+   virtio/index
    vme
+   w1
+   xillybus
+
+
+Subsystem-specific APIs
+=======================
+
+.. toctree::
+   :maxdepth: 1
+
    80211/index
-   uio-howto
+   acpi/index
+   backlight/lp855x-driver.rst
+   clk
+   console
+   crypto/index
+   dmaengine/index
+   dpll
+   edac
    firmware/index
-   pin-control
+   fpga/index
+   frame-buffer
+   aperture
+   generic-counter
    gpio/index
+   hsi
+   hte/index
+   i2c
+   iio/index
+   infiniband
+   input
+   interconnect
+   ipmb
+   ipmi
+   libata
+   mailbox
    md/index
    media/index
+   mei/index
+   memory-devices/index
+   message-based
    misc_devices
+   miscellaneous
+   mmc/index
+   mtd/index
+   mtdnand
    nfc/index
-   dmaengine/index
-   slimbus
-   soundwire/index
-   thermal/index
-   fpga/index
-   acpi/index
-   auxiliary_bus
-   backlight/lp855x-driver.rst
-   connector
-   console
-   eisa
-   isa
-   io-mapping
-   io_ordering
-   generic-counter
-   memory-devices/index
-   men-chameleon-bus
    ntb
+   nvdimm/index
    nvmem
    parport-lowlevel
+   phy/index
+   pin-control
+   pldmfw/index
    pps
    ptp
-   phy/index
    pwm
-   pldmfw/index
+   regulator
+   reset
    rfkill
+   s390-drivers
+   scsi
    serial/index
    sm501
+   soundwire/index
+   spi
    surface_aggregator/index
    switchtec
    sync_file
+   target
+   tee
+   thermal/index
    tty/index
-   vfio-mediated-device
-   vfio
-   vfio-pci-device-specific-driver-acceptance
-   virtio/index
+   wbrf
+   wmi
    xilinx/index
-   xillybus
    zorro
-   hte/index
-   wmi
-   dpll
-   wbrf
-   crypto/index
-   tee
 
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