Fix reference in rst to markdown files

[arnopo]

In rst file to refer to another rst file we can use :ref: tag. in such case the reference point to the generated documentation page.
But we can not refer to markdown file in the same way.

For instance in https://raw.githubusercontent.com/OpenAMP/openamp-docs/refs/heads/main/demos/system_reference-NXP.rst

Initialize the Zephyr environment
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Refer to `zephyr example readme
<https://github.com/OpenAMP/openamp-system-reference/blob/main/examples/zephyr/README.md#initialization>`_
article.

we should be able to refere to a page in the generated documentation instead of pointing to the github address of the file.

This is created following discussions in #56

[sipke]

Investigating this, using cross references one can reference between rst and md files in any combination.
One can also enable auto generated references as per this link.

Using either or both of those works nicely from openamp-docs sphinx generated documents to source repository files.

However things break if trying the same links between source repository md or rst files as github rendering does not match sphinx feature set.

• All links created/generated by github break
• The manually added references result in github generating a spurious 'tag' in the markdown file, for example (echo_test_run_the_demo)=

To avoid the second problem, the plan is to setup openamp-docs to have auto generated references (this avoids having to add manual reference 'tags' inside the source repository markdown files, so github rendering will not be affected).

To avoid the first, we only use references to source repository 'auto generated references' from openamp-docs. This just means that any references between source repository documentation needs to use the URL, as they are now. There is at least one here.

See #65