Add `ocpi_documentation_include` directive to ocpidoc

What this MR does / why we need it:

This merge requests adds a custom include directive to ocpidoc that behaves similar to the Sphinx ".. include" directive but also allows for "find and replace" functionality. Ultimately this enables figure and equation numbering in included common documentation without Sphinx complaining about label duplication.

An example of the directive's use is shown below:

   .. ocpi_documentation_include:: ../common/shared_section.inc

      |component_name|: polyphase_clock_synchroniser_xs
      |path_to_figures|: ../common/

Which allows content like the following, within the included file:

   This problem is shown in :numref:`|component_name|-sampling-points-diagram` below.

   .. _|component_name|-sampling-points-diagram:

   .. figure:: |path_to_figures|symbol_sampling_points.svg
      :alt: Symbol Sampling Points.
      :align: center

      Symbol Sampling Points

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Release Notes

N/A

Changelog

• enh(tools,documentation): add ocpi_documentation_include directive to ocpidoc

Which issue(s) this MR closes

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