Documentation: use a source-read extension for the index link boilerplate

The root document usually has a special :ref:`genindex` link to the
generated index. This is also the case for Documentation/index.rst. The
other index.rst files deeper in the directory hierarchy usually don't.

For SPHINXDIRS builds, the root document isn't Documentation/index.rst,
but some other index.rst in the hierarchy. Currently they have a
".. only::" block to add the index link when doing SPHINXDIRS html
builds.

This is obviously very tedious and repetitive. The link is also added to
all index.rst files in the hierarchy for SPHINXDIRS builds, not just the
root document.

Put the boilerplate in a sphinx-includes/subproject-index.rst file, and
include it at the end of the root document for subproject builds in an
ad-hoc source-read extension defined in conf.py.

For now, keep having the boilerplate in translations, because this
approach currently doesn't cover translated index link headers.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
[jc: did s/doctree/kern_doc_dir/ ]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260123143149.2024303-1-jani.nikula@intel.com>

Author: jnikula

Documentation/RCU/index.rst

@@ -28,10 +28,3 @@ RCU Handbook
    Design/Expedited-Grace-Periods/Expedited-Grace-Periods
    Design/Requirements/Requirements
    Design/Data-Structures/Data-Structures
-
-.. only:: subproject and html
-
-   Indices
-   =======
-
-   * :ref:`genindex`

Documentation/accel/index.rst

@@ -11,10 +11,3 @@ Compute Accelerators
    amdxdna/index
    qaic/index
    rocket/index
-
-.. only::  subproject and html
-
-   Indices
-   =======
-
-   * :ref:`genindex`

Documentation/admin-guide/aoe/index.rst

@@ -8,10 +8,3 @@ ATA over Ethernet (AoE)
     aoe
     todo
     examples
-
-.. only::  subproject and html
-
-   Indices
-   =======
-
-   * :ref:`genindex`

Documentation/admin-guide/auxdisplay/index.rst

@@ -7,10 +7,3 @@ Auxiliary Display Support
 
     ks0108.rst
     cfag12864b.rst
-
-.. only::  subproject and html
-
-   Indices
-   =======
-
-   * :ref:`genindex`

Documentation/admin-guide/cgroup-v1/index.rst

@@ -22,10 +22,3 @@ Control Groups version 1
     net_prio
     pids
     rdma
-
-.. only::  subproject and html
-
-   Indices
-   =======
-
-   * :ref:`genindex`

Documentation/admin-guide/cifs/index.rst

@@ -12,10 +12,3 @@ CIFS
    todo
    changes
    authors
-
-.. only::  subproject and html
-
-   Indices
-   =======
-
-   * :ref:`genindex`

Documentation/admin-guide/device-mapper/index.rst

@@ -40,10 +40,3 @@ Device Mapper
     verity
     writecache
     zero
-
-.. only::  subproject and html
-
-   Indices
-   =======
-
-   * :ref:`genindex`

Documentation/admin-guide/gpio/index.rst

@@ -12,10 +12,3 @@ GPIO
     gpio-sim
     gpio-virtuser
     Obsolete APIs <obsolete>
-
-.. only::  subproject and html
-
-   Indices
-   =======
-
-   * :ref:`genindex`

Documentation/admin-guide/index.rst

@@ -189,10 +189,3 @@ A few hard-to-categorize and generally obsolete documents.
 
    ldm
    unicode
-
-.. only::  subproject and html
-
-   Indices
-   =======
-
-   * :ref:`genindex`

Documentation/admin-guide/kdump/index.rst

@@ -11,10 +11,3 @@ information.
 
     kdump
     vmcoreinfo
-
-.. only::  subproject and html
-
-   Indices
-   =======
-
-   * :ref:`genindex`