[PATCH rtems-docs] user: Update @rtems-ver-major@ to use sphinx substitution extention

[Chris Johns]

On 18/7/2023 6:16 pm, Muhammad Sulthan Mazaya wrote:
>  Using sphinx-substitution-extensions to manage the variable @rtems-ver-major at . 
>  Originally Utkarsh's idea: 
>  https://discord.com/channels/820452222382112799/820452222848335924/1118416996606943302
>  
>  I have tested and built the docs to see if it works and it does 
>  subtitute the variable. The changes are: the variable @rtems-ver-major@ 
>  is now |rtems-ver-major|. For inline code block we use 
>  :substitution-code:`{content}` and for multiline code block we use:
> 
>  .. code-block:: {language}
>    :substitutions:
>    
>    {content}

This is a welcome update to the documentation. Thank you for looking into it.

Does this patch require any additional sphinx modules to be installed?

Chris