Include README.rst in doc/ files

[Andy-Grigg]

Description of the modifications

https://github.com/ansys/openapi-common/blob/main/doc/source/index.rst and https://github.com/ansys/openapi-common/blob/main/README.rst share a significant amount of content, but this content is duplicated between the two. Additionally, there is content in the README.rst which does not appear in the HTML document, e.g. the section on HTTP certificates

Useful links and references

No response

[Andy-Grigg]

We have just done a new release for this, but we could do a post or a patch release to include this.