Update Documentation to Include Public API Endpoint

[benefacto]

I noticed that the documentation (API, website) does not include the public API endpoint for OnFinality.

Current Situation:

• The endpoint api.onfinality.io is not documented anywhere.
• No information on sandbox or testing endpoints if they are offered.

Workaround:
To find the endpoint, I had to capture and analyze network traffic using the following steps:

1. Capture network traffic using tcpdump:

   sudo tcpdump -i any -w api_traffic.pcap 'tcp port 80 or tcp port 443'

2. Run an arbitrary CLI command, such as:

   onf node list

3. Analyze the capture file with tshark:

   tshark -r api_traffic.pcap -Y ssl.handshake.extensions_server_name -T fields -e ssl.handshake.extensions_server_name

   This revealed the endpoint: api.onfinality.io.

Request:
Please update your documentation to include the public API endpoint api.onfinality.io. Additionally, if there are sandbox or testing endpoints available, including those would greatly benefit developers.

Additional Context:
I am creating this issue here because your two public documentation repositories https://github.com/OnFinality-io/documents (last updated 3 years ago) and https://github.com/OnFinality-io/documentation (last updated 2 years ago) seem out-of-date or unmaintained. Documenting this information anywhere would be better than nowhere, even if it's just in the CLI repository, as CLI developers may need to drop down into the API directly for some tasks.