Update Open-API docs with authentication info

[kievitsp]

The Open-API documentation is updated to include important information regarding the requirement of authentication to access the rest endpoints. This information is crucial for developers and users integrating and interfacing with the API, helping them understand and ensure proper authentication to avoid potential access issues.

Thank you for contributing to the documentation.

Do the changes you have made apply to both Current and Previous versions?

Have you checked all new or changed links?

Is there anything else you would like us to know?

For reference:

• We have an internal contributions guide
• We have a style guide

This week's exciting excerpts from the style guide

• We write in UK English, with Oxford English Dictionary (OED) spellings, grammar and vocabulary.

Use the present tense wherever possible. Only use another tense where it is strictly necessary.

• Present tense (preferred whenever possible): The script creates a new folder.
• Future tense (avoid whenever possible): The script will create a new folder.