[DOCS]: Add "Roles" page to branch 7.0

Description

We need to move the content from legacy repo and convert it to RST, as well as update the content.

[!IMPORTANT]
Please read the below resources before contributing to help you with this task:

• Contributing guidelines
• Contributing to Mautic’s documentation
• Mautic technical writing style guide
• The "Suggestions (Optional)" section below for guides on how to update the page
• Mautic API Library

Relevant Documentation URL

https://devdocs.mautic.org/en/7.0/rest_api/roles.html

Link to the content at legacy docs

https://github.com/mautic/developer-documentation/blob/main/source/includes/_api_endpoint_roles.md

Are you willing to help us fix this by making a pull request?

No

Suggestions (Optional)

How to update the page

• Find the file that needs to be updated in the docs/ directory. The dev docs URL format is as such:

  https://devdocs.mautic.org/en/x.x/FOLDER_NAME/FILE_NAME.html
  

  For example, if the relevant URL is https://devdocs.mautic.org/en/7.0/plugin_extensions/manipulating_contacts.html, then find manipulating_contacts.rst file inside the plugin_extensions folder.

• Transfer the content under the note admonition. Look at other pages on how to structure the content.

• Review and update the content when you see fit.

• Follow the contributing guidelines that are listed in the "Resources" section above.

Screenshots or Screen Recordings (Optional)

No response

Read the contributing and technical writing resources, then compare the legacy content in source/includes/_api_endpoint_roles.md with the target page at docs/rest_api/roles.rst, based on the 7.0 URL. Transfer the content into the RST page, update it as appropriate, and verify that the Roles page is complete at the documented URL.