[DOCS]: Add "Notes" 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/notes.html

Link to the content at legacy docs

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

Start with the legacy content in source/includes/_api_endpoint_notes.md and the contributing and technical writing guidance linked in the issue. Create or update the corresponding docs/rest_api/notes.rst page, convert the content to RST, and revise it as appropriate; done means the Notes page is complete at the 7.0 REST API URL.