update(rest api): Add authentication method in REST API

[Meggielqk]

Update the REST API document according to the Technical Support's feedback recorded in this issue: https://emqx.atlassian.net/browse/EDOC-340?atlOrigin=eyJpIjoiOGRiNDkyZWQ0MGY1NDZkMWEzYTc3ZDFiNWU5MTkxMjQiLCJwIjoiaiJ9.

The main changes are:

• A new authentication method "authentication with bearer token" is added.
• The "HTTP Headers" and "HTTP Response Status Code" sections are moved before authentication methods so that users understand how the API communicates before setting up authentication.

[Benniu]

To get the Bearer token, just need to send an HTTP request to call the login API. You only need to clearly describe this API endpoint, or link directly to the login API documentation. Examples with curl or shell commands are unnecessary, as these are just two of many tools for calling an API. Other tools, like Postman, or even programming methods, can also be used to make the API call. So, the main focus here is to describe the login API interface clearly, not to show how to use a specific tool to call it.