Document new CMS settings

[timber-they]

With #440 new settings appeared:

# CMS contents
CMS_API_BASE_URL = ''
CMS_API_BASE_PATH = ''
CMS_API_KEY = ''

However, they are not documented in our README.md. If not configured properly, the API throws cryptic errors though:

api-1            | Internal Server Error: /cms/page/slug/
api-1            | Traceback (most recent call last):
api-1            |   File "/usr/local/lib/python3.8/site-packages/django/core/handlers/exception.py", line 47, in inner
api-1            |     response = get_response(request)
api-1            |   File "/usr/local/lib/python3.8/site-packages/django/core/handlers/base.py", line 181, in _get_response
api-1            |     response = wrapped_callback(request, *callback_args, **callback_kwargs)
api-1            |   File "/badgr_server/apps/mainsite/views.py", line 734, in cms_api_page_details
api-1            |     api_data["post_content"] = cms_transform_urls(api_data["post_content"])
api-1            | TypeError: string indices must be integers
api-1            | 2025-08-02 01:54:37,416 ERROR log Internal Server Error: /cms/page/slug/
api-1            | Traceback (most recent call last):
api-1            |   File "/usr/local/lib/python3.8/site-packages/django/core/handlers/exception.py", line 47, in inner
api-1            |     response = get_response(request)
api-1            |   File "/usr/local/lib/python3.8/site-packages/django/core/handlers/base.py", line 181, in _get_response
api-1            |     response = wrapped_callback(request, *callback_args, **callback_kwargs)
api-1            |   File "/badgr_server/apps/mainsite/views.py", line 734, in cms_api_page_details
api-1            |     api_data["post_content"] = cms_transform_urls(api_data["post_content"])
api-1            | TypeError: string indices must be integers
api-1            | [02/Aug/2025 01:54:37] "GET /cms/page/slug/?slug=startseite HTTP/1.1" 500 22320
api-1            | Internal Server Error: /cms/page/slug/
api-1            | Traceback (most recent call last):
api-1            |   File "/usr/local/lib/python3.8/site-packages/django/core/handlers/exception.py", line 47, in inner
api-1            |     response = get_response(request)
api-1            |   File "/usr/local/lib/python3.8/site-packages/django/core/handlers/base.py", line 181, in _get_response
api-1            |     response = wrapped_callback(request, *callback_args, **callback_kwargs)
api-1            |   File "/badgr_server/apps/mainsite/views.py", line 734, in cms_api_page_details
api-1            |     api_data["post_content"] = cms_transform_urls(api_data["post_content"])
api-1            | TypeError: string indices must be integers
api-1            | 2025-08-02 01:54:37,416 ERROR log Internal Server Error: /cms/page/slug/
api-1            | Traceback (most recent call last):
api-1            |   File "/usr/local/lib/python3.8/site-packages/django/core/handlers/exception.py", line 47, in inner
api-1            |     response = get_response(request)
api-1            |   File "/usr/local/lib/python3.8/site-packages/django/core/handlers/base.py", line 181, in _get_response
api-1            |     response = wrapped_callback(request, *callback_args, **callback_kwargs)
api-1            |   File "/badgr_server/apps/mainsite/views.py", line 734, in cms_api_page_details
api-1            |     api_data["post_content"] = cms_transform_urls(api_data["post_content"])
api-1            | TypeError: string indices must be integers

So I assume it's important to properly configure those parameters. Since they're not documented at all and the log messages are also not helpful, I don't know how to configure them though.