Merge branch 'feature/build_docs_h21' into 'master'

docs(build): add support for building H21 docs

Closes IDF-12105

See merge request espressif/esp-idf!36632

docs/en/api-reference/peripherals/gpio/esp32h21.inc

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+.. This file gets included from other .rst files in this folder.
+.. It contains target-specific snippets.
+.. Comments and '---' lines act as delimiters.
+..
+.. This is necessary mainly because RST doesn't support substitutions
+.. (defined in RST, not in Python) inside code blocks. If that is ever implemented,
+.. These code blocks can be moved back to the main .rst files, with target-specific
+.. file names being replaced by substitutions.
+
+.. gpio-summary
+
+  To be updated
+
+.. note::
+
+  To be updated
+
+---

docs/en/api-reference/system/inc/espefuse_summary_ESP32-H21.rst

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+.. code-block:: none
+
+   To be updated

docs/en/api-reference/system/inc/espefuse_summary_ESP32-H21_dump.rst

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+.. code-block:: none
+
+    To be updated

docs/en/api-reference/system/inc/power_management_esp32h21.rst

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+
+.. flat-table::
+    :widths: 1 3 3
+
+    * - Max CPU Frequency Set
+      - Lock Acquisition
+      - CPU and APB Frequencies

docs/en/api-reference/system/inc/revisions_ESP32-H21.rst

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+
+.. list-table::
+    :header-rows: 1
+    :width: 2 5
+    :align: center
+
+    * - ECO
+      - Revision (Major.Minor)

docs/en/api-reference/system/inc/show-efuse-table_ESP32-H21.rst

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+
+.. code-block:: none
+
+    To be updated