Mnesia authentication docs mentions non-existend api endpoint

Hi,

since `emqx_auth_clientid `and `emqx_auth_username` are merged to `emqx_auth_mnesia` starting with 4.3.0 we followed the [Mnesia Authentication](https://docs.emqx.io/en/broker/v4.3/advanced/auth-mnesia.html) docs to create users by using the api, which is the more secure way instead of creating them via `emqx_auth_mnesia.conf` file.

Unfortunately, the docs mention to use the `api/v4/auth_user` endpoint, which returns `404 Not Found`. We checked the source of the plugin and identified a different path `/auth_username` is used by this plugin (see original source -> [LINK ](https://github.com/emqx/emqx/blob/186094d2abf33f8bc081bbbb74683669510ab374/apps/emqx_auth_mnesia/src/emqx_auth_mnesia_api.erl#L77-L82) )

In https://github.com/emqx/emqx-docs/issues/795 someone already mentioned that the endpoint from docs does not exist. Commit 39feb21d4dfb8b6ff86bd78b02539e03ade142fc already fixed the documentation, but it is not yet published on docs.emqx.io.

File: [/en_US/advanced/auth-mnesia.md](https://docs.emqx.io/en/broker/v4.3/advanced/auth-mnesia.html)

Regards,
Alex

Open /en_US/advanced/auth-mnesia.md and compare its API endpoint with the emqx_auth_mnesia source linked in the issue. Review commit 39feb21d4dfb8b6ff86bd78b02539e03ade142fc and verify whether the corrected documentation is published at docs.emqx.io; done means the live page uses the existing endpoint.

Written by the indexing model from the issue text.